If you know what you are doing, you can do it from the shoulder too. Belt loop is easiest to learn though.
I used to shoot out in the woods near an overgrown old town sand pit. I LOVED bump-firing the AK. (I won't do that with my $800 AR, though). Some guy lived on a farm about a mile from there, he'd always answer back either by bump firing or full auto. ;-)
Anyway, eventually I decided I had to "get serious" and joined a shooting club. My first question: "Can I bump fire my AK here?" got a quizzical look. Eventually, after some confusion I founf the part in the club rules related to what they call "Casual Rapid Fire," which is as close to bump firing as they seemed to understand - but the rule for that was no more than 6 rounds to a magazine at a time.
What is the point in that?