I think I was the one who mentioned it. I've read about it, but not tried it.

I believe you want a basal cutting, either with all the leaves cut off and just the terminal leaf/buds remaining, or with most of the leaves cut off and the terminal leaf/bud also cut off. Basically, you want to limit transpiration through the leaves since there are no roots to bring up moisture. Some people say clipping the terminal leaf/bud is a good idea, too, so the plant concentrates on roots rather than upward growth, but where I read about big root geranium they kept the terminal leaf.

Then you want to put it in some good, rich soil, water well, and (according to some sources) give it bottom heat. Spring is the best time, but you could try it now. I wouldn't take a stem with flowers on it, though-- too much energy going to that. Make sure you mist it a few times a day to help it retain moisture, or I've sometimes seen people bag the pot to create a high-moisture tent over the cuttings.

This lady is doing basal cuttings on delphinium, I believe, but it gives the general idea. I wouldn't leave as many leaves as she does, though. http://www.gardenersworld.com/how-to/projects/creative-projects/how-to-take-basal-cuttings/24.html

Hope that helps! I was going to wait until next spring, but I think I'll give it a go this year.

Source: https://www.rhs.org.uk/plants/details?plantid=862

Big root geranium, the straight species, and the related hybrids--Bevan's variety, Karmina, Biokovo, etc-- are probably the easiest plants in the world to get starts from. Simply reach in and pull out one of the long roots with foliage attached. Stick it in the ground in a long shallow planting. Dump water on it.I have planted hundreds like this, and given away dozens with the same instructions. Even in the heat of summer without any subsequent watering, most survive. it's simply not a major production as described above.

Agree with laceyvail - just have at it. They'll be fine. I just transplanted a few Biokovo yesterday - reached down, yanked, and transplanted the clump of roots that pulled right up.

Thanks! So, I do need to get roots. Sorry if
I seem obtuse: just don’t want to mutilate my plant without results. I have a big
area to fill and there would not be much left of the parent plant. The one I have has sort of a rounded dome of
foliage, then closer to the bottom are chunky stems with dried up leaf stems on
them. What I wasn’t clear on was whether I just needed a section of bare (ish)
stem, or if I needed to go all the way down to the base and get a section of
the root.

linnea...

..here is a photo I've just taken of my 'Bevan's Variety' that I've removed a shoot from, right from the base of my plant.... I've got so many of these that missing this one won't hurt...

...I've yanked the stem right from under the soil so as to get a bit of root that you can see... what I would do here is trim off all the flowering stems, leaving just the brown woody root piece with some green shoots coming out the top, and then bury that piece in the soil, water in, with just the green shoots showing above ground...

....if I really wanted to, I could probably get maybe 3 plants out of that one piece of root, cutting it up and burying each one, leaving a tip exposed above ground.... it's surprising how quickly they form shoots at this time of year.... although occasionally one or two will die off.... but if you leave that root cutting as one plant, it should take really well....

Wow! Thank you so much, Marlorena! You have taken all the mystery out of it!
I think I’ll try it both ways: as one long piece, and a piece cut up into
sections.

I ended up just using a shovel to split mine, as in the bull in a china shop method I mentioned on the other thread. They all look great.

I still have more to divide and am definitely going to try cutting the roots into sections like Marlorena. Lots of space to fill. Thanks for the pictures!

Thank you Laceyvail and Malorena for this post. I thinned a clump of Bevan's Variety yesterday and used the roots to underplant shrubs at the historic site. If even just a third of them grow it will look marvelous and "yank and plant" was very fast.