i believe the finished compost falls throught the grates when the worms move through the compost and eat up the bottom layers. basically it's like a sifter really. the worms will eat up the materials and poop thus less material as it builds up some parts start to give away and the compost that is done will filter through the grates and into the bottom container thus making it extremely easy on your part to harvest.

their may be some material that has not been fully composter and may has some worms.

if you are building a flow through bin i that is tall i would suggest using it like a worm tower. add a small layer add the worm the food etc.. as they start breaking that down add more layers so they migrate up and the finished stuff goes though the grates and you can harvest it. also the weight from the new layers will push the finished stuff down thanks to gravity. depends on what worms you got burrowing worms i see no problem if you just fill it up all at once but things like red wigglers whom mainly stay near the surface obviously they won't harvest the stuff on the very bottom if your using like a 5 gal bucket.

and remember they like surface area rather then depth. burrowing worms like depth but if your using burrowing worms you would want more soil then paper from what i heard

"Why are they so much more effective than a plain worm bin?"
I don't know that they're more effective. They can be easier to operate as it's a continuous process - waste in a the top and finished product out the bottom. This is an attractive alternative to 'batch' processing with other designs.

I'd say easier maintance and general upkeep as opposed to 'effective'. Bins are quite effective in creating VC. Maybe you can push more stuff through a FT just because it's more forgiving.

But some of the pros of a flowthrough.

Better aeration, can come from the bottom, it's like a huge vent.

No soggy bottom problem. Being drier at the bottom means nicer VC when you harvest and also better airflow again. Less worry about food wastes with lots of water.

Never have to do a full harvest and no large seperation of worms.
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Harvesting can be somewhat tricky though. Unless you have a harvest system (harvest bar, scraper etc.) having it flow through by itself past a grating can be a bit hit and miss. I use a Worm Inn and even with the bottom wide open, I've still got to dig with a hand fork to get stuff out. But that's also a good thing as you don't risk having the whole lot drop to the ground heh.

"I use a Worm Inn and even with the bottom wide open, I've still got to dig with a hand fork to get stuff out." Tell me that ain't so. I envisioned the stuff just falling out after squeezing the sides a bit. Maybe it is just like in the plastic sided flow throughs and neither the bins nor the inns have read the instruction manual.

Another supposid benifit of a flow through is not needing to lift up the bin to harvest.

The VC falls through the bottom grate when its cohesion with surrounding VC is less than the force of gravity pulling it down.

The problem that I've seen with them is that the bottom layers harden into a brick so additional effort is required to break clumps loose. Often this will not result in the VC above the broken zone settling into the newly opened area so you get a hardened dome of VC that requires you to get up under it and smack it with something until it falls. When it does fall, keep in mind that it may have enough momentum to break the bottom screen and dump the bin on the ground, or you.

Being crushed by a pile of dirt and/or manure is a guaranteed placement in the Darwin awards.

Taking your question of why they are more "effective" literally, that idea is silly. They don't make a better product faster, or with less work. The most effective would probably be the windrow system, as that is what is done commercially.

The windrow system adds material to the end and allows the worms to move though it. When the worms complete an area and that area matures, it is collected.

This windrow system can be replicated in typical horizontal bins and the stack of trays style bins, but is not well implemented in the 'drop through' flow through systems. The drop through systems essentially move the worms out of a section when that section becomes too dry for them to survive there, regardless of how ready the compost is. It then turns into a hard brick, etc, etc. In a bin where the compost is kept habitable until it is collected, you can be sure that the area that both looks done and is not inhabited by worms is likely to be the closest to finished and is quality "effective" VC.

"Tell me that ain't so. I envisioned the stuff just falling out after squeezing the sides a bit. Maybe it is just like in the plastic sided flow throughs and neither the bins nor the inns have read the instruction manual.
Another supposid benifit of a flow through is not needing to lift up the bin to harvest.
"

Heh, well you CAN squeeze it if you've got some strength in your arms (it's heavy stuff), but it's a risk. The last thing you want is all your VC, your recent feedings and worms pouring out the bottom. So what I do is a scraping harvest until I get all I want. Then I shut the bottom and will squeeze it to breakup the VC and have it drop down and also aerate it to the max.

One thing you can do with a FT is with a bucket at the bottom give it a large watering. That will soften the VC and allow for an easier harvesting. Not too much of an issue with the Worm Inn as it's a short system and the bottom doesn't turn into concrete, but does get compacted. But nothing a raking with a fork won't harvest. But that's more system dependant.

Very in formative, what is the FT? I have started a worm bucket. The first time I has worms excaping. Now I have a few and seems to be doing okay. I'm just getting the worms out of the yard.

FT is just short for Flow Through. The one I have is called the Worm Inn.

The worms from the yard are probably not composting worms unless you're really lucky. Earthworms don't really work for composting food. Here's a great site on worms and composting.
http://www.redwormcomposting.com/getting-started/

Many people start composting with a bin. Then they start to notice all the good stuff is on the bottom. So they started cutting a hole in the bottom to get at the good stuff while still filling the top. Then someone added bars across the bin to better access the vermicompost from the bottom. In theory this should work. However, it does not always. Sometimes the access hole is too large and the thin walled plastic container colapses into a heap. Other times after the first harvest the material does not seem to want to follow the rules of gravety and fall down to the bars to harvest. Other times it falls down too much so vermicomposters line the bars with paper or cardboard and let that rot away and start the flow through process. Other than profesional systems and a few posters here many of us are still strugling to get our systems to that balanced point of perfect vermicompost harvesting in a flow through system. Theoretically it exists and should be possible. Then along came the Worm Inn which had soft breatheable sides and a tie closure at the bottom. I really wanted one of those, and still do. But now I hear that that too is not perfect and subject to some of the same issues hard sided flow Throughs are. Many of us here are having fun trying to perfect the flow through system whether hard or soft sided. After getting the basics of keeping the worms alive the how to get the flow through to... flow through keeps us very occupied. Many of us are experimenting building a better mouse trap or better flowing flow through. We are happy doing this work.

I have two FT systems and may start a third soon.

First I started with a worm inn. I like this one but I find that it dries out really fast and I have to add about a quart of water almost weekly even when I am really feeding it hard.

I then built a wooden FT with white wire closet shelving as the bottom. This box is about 24" x 12" horizontally. This was only started about 2 to 2.5 months ago. I have been feeding it hard and it seems to have taken off well. When I started this one I split my nearly full worm inn in half to have the two bins.

I have been adding a lot of veg waste weekly - well over 10 pounds - between the bins. Lately I have also started to add veg garden waste too. Perhaps I am pushing them harder than I should as I have not read about anyone else pushing theirs this hard aside from Bently at redwormcomposting.com.

I have not yet harvested from the new wooden bin but the lower reaches look really good when I dig around in it.

The worm inn knock off I have will dump out of the bottom but it requires coaxing out and maybe a few punches to the bag along the way. Don't expect perfect fine grains of compost from this directly out of the bottom especially if you constantly feed it. Rather I have had to sift the compost each time I removed it.