The AR has the gas rings and pressure behind the bolt because it has a lot of lug area, and the lugs are flat with no cam angle. To get it to unlock while there's still adequate pressure available, the gas into the AR bolt briefly reduces the load on the lugs.
The AK also has a flat lug bearing surface, but much smaller in area than the AR, and the flats radius down into opening cams on the lugs. You don't have to force the AK bolt completely out of alignment with the trunnion; you just have to kick if off the flat spot and it'll open itself.
By comparison, my Ross bolt-action rifle has no flat spots at all - the receiver and bolt head are both machined at a shallow angle. It doesn't blow open during firing because the friction from the lugs mating is enough to keep the bolt from opening, sort of like the friction of your tires keeps your car from sliding down a hill if you park on an incline. (unless you live in Virginia, where any lot with less than an 80 degree slope is considered "flat") The point where friction is overcome by load is called the "angle of repose".
The Ross is designed that way to make the bolt easier to open, because it uses a straight-pull design that has poor primary extraction leverage. And though it's not relevant to the concept of a direct impingement AK, I spent hours staring at bits of that rifle laid out on my desk wondering "WTF?" and wanted to share it with someone...