When you soak your handspun yarn in warm-to-hot water, it brings back the 'memory' of the yarn and lets it relax. If you've spun your yarn very tightly (or even a bit too tightly), then it is likely to seem to relax in the hot water. If you spun it and then plied it, and got it 'exactly right', then it likely wouldn't seem to relax when put in hot water, since all of the fibers will come into natural alignment and find themselves happy. A 'balanced' ply has all of the fibers (ok, most?) running parallel to the length of the yarn.
(ed. note: the picture is some of my "crazy yarn", spun in 2006-2007!)
You can use this hot water technique to figure out what the right twist was for plying yarn -- freshly spun singles can show you what the right amount of plying-twist is, but once they've sat, even for a few hours, the twist begins to set in them. So, I take a sample when it's fresh, let it twist on itself, and stick it to my sample card. Then I let the bobbins (or, spindles) sit for a weekend (or a week! sigh!) so the twist is a bit set and the singles are less lively, and ply on this semi-set stuff to the same twist that the live sample showed me. If you forgot to sample before letting it sit, take a length of singles, drop it in hot water, and eureka! like magic, it will twist on itself and show you how it should be plied.
(posted by me on spindlers, 12/12/2001)