I don't know the science behind it, but if you squirt it on something and watch closely, you can actually see it "crawl" around for lack of a better word.
I think the acetone lowers the surface tension of the ATF allowing it to penetrate.
There was a thing on the internet several years ago about a few ounces of acetone in the gas tank lower the surface tension of the gas allowing it to break up into smaller droplets blah blah. Well, I tried it on my Daughters 75 Mustang once, the weather was pretty cool, and after that she was on me constantly to put the acetone in her car. she said it started and ran so much better until it was up to temp. Of course it had a carb, everything else was fuel injected and I never noticed squat on my car.
Ron.