I'd recommend a book called "Botany for Bonsai...the science behind the art", by Enrique Castano de la Serna, which is fascinating as he explains a lot of the botany behind the bonsai techniques we apply. Castano has a Phd in biochemistry and biophysics, and is a bonsai artist, so it's quite a perspective for an avowed "bonsai idiot" like me!
Long-Short to your question: Your goal is always to preserve the tree's energy. Energy is manufactured by leaves and ultimately stored in roots & heartwood. In the fall, all of the energy (starches and sugars) moves from the top of the tree down into the root system and the top becomes dormant. Since the energy isn't in the top now, it's safe to prune and wire the branches in the winter. In the spring, all of that energy stored in the roots makes a big push up into the remaining buds, and leaves are manufactured. Since the energy isn't in the roots anymore, it's safe to prune them and repot.
Look at a tree like a half-inflated balloon that you have gripped in your fist like an hourglass, and the energy is the air inside the balloon. When you force all the air into the top half of the balloon (simulating spring time growth), you can cut the bottom of the balloon and still have air (energy) in the top half; that's like springtime root pruning. The metaphor is applicable in all other scenarios; some of which result in the balloon popping (tree dies).
If you prune roots now, you remove much of that stored energy, and the tree has less energy to push up into the buds in the springtime. Assume it has enough to make some attempt at growing, and then you start pinching growth, you're again removing it's capacity to generate sugars/starches, and less energy is stored in the root system...downward death spiral for trees.