I'm not sure I understand what you mean, but if you dig up the soil around your trees, bushes, or vines to the depth of half a spade (think that's kind of what you meant), you'll be digging up a good portion of the roots! Major root disturbance is not helpful for the growth of your plants and is also a potential way to introduce disease. Keep that shovel in the garage until it's time to plant another tree.

Hm, here what Russian said:
"In late September we do shallow digging in the soil between rows and under fruit-bearing trees and shrubs. Plast land flip, but not split. Creating cloddy surface under winter contributes to better water retention, it gets frozen better, so that the larvae of pests that overwinter in the soil, dying."

Briergardener,

For the most part this is a very arcane practice. It used to be done here generations ago, but has largely been abandoned.

There are a few insects pests that pupate in the soil, as well as some diseases that over-winter in leaf litter. While tillage can destroy some of the larva, and bury the leaf litter, in most places the benefits of tillage are not worth the costs (erosion, damaged roots, reduces organic matter, etc.).

It was practiced by our forefathers primarily because they had few options for pest control at the time. It was a practical way to control weeds.

BTW, cloddy soil doesn't retain moisture better, it dries out faster. I do agree tilled soil gets colder in the wintertime by removing plant residues from the top of the soil (which act as an insulator) and by creating a break at the bottom of the tillage zone (which reduces heat conduction from the warmer soil below).

However, this is a moot point in zone 7. Your winters don't get cold enough to kill any soil borne pests. Additionally, the "big three" insects pests east of the continental divide (Plum Curculio, Codling moth, and Oriental Fruit moth) all overwinter above the soil, and can take amazingly frigid temperatures. I've read Codling moth can take -40F. Plum Curculio readily survives in zone 4 (-30F).

On a list of fall chores, spading under trees wouldn't make the top 1000 for me.

Disturbed soil can sometimes absorb rain better, so maybe that's where the thought came from. That sounds like a terrible practice- loosening the soil just before winter would create huge erosion issues.

The idea of digging under bearing trees may be a way of reducing vigor and ultimately improving fruit quality. I bet it would have some benefit in some instances. Root pruning is a common practice for fruit trees in many parts of the world. Be better to do it in Spring if you wanted to experiment. I just may try it on one of my peach trees.

I root pruned a Japanese plum last season, but by cutting specific large roots and got some benefit I think.