Here's an attempt at an answer nearly a year late. I'm a brainlet at physics and this is probably completely wrong.
I think it comes down to that when you're accelerating your car is going to be pushing forward with more force, making the turn more rigid - the "forward" of accelerating is fighting your turning motion - and ultimately causing you to have to swoop around in a larger arc. Decelerating (not necessarily braking, just letting off the gas) means the turn can be tighter, so you spend less time swooping around, and then you floor the gas. Economy of motion, kind of? You're trying to avoid wasting time making larger motions than necessary or going slower (as if you hugged the in side) than necessary.
I just know that when I started doing that in racing games I got a lot better. At the same time, I found that drifting around a turn but then flooring the gas could also "straighten" my car out as the turn concludes, which was very useful.