I can only really explain it intuitively, in that when you pull on the rear brake or engine brake, the angular forces of the rotation of the wheel are transferr

I can only really explain it intuitively, in that when you pull on the rear brake or engine brake, the angular forces of the rotation of the wheel are transferred into making your bike straighten up vertically. I don't know if that actually happens, but that's how it feels. Part of it is also similar to the reasons you trail brake into corners, in that you're managing the weight distribution over your tires. You don't want to be erratically changing how much weight is on the front tires, otherwise you'll find you suddenly have a lot less front grip.