Back foot flips are definitely on the advanced side, and not easy to learn. There are no legitimate prerequisite tricks to learning it, but very good board control and balance is necessary, as well as a solid ollie. The idea is to ollie, and at the peak, right as you start to descend, flip it with your back foot. Since it is flipping on the way to the ground, it is harder to catch, and being able to get the board to flip is definitely the hardest part.
Good board control and balance, easily able to control where to flick.
Doing The Trick
Positioning:
Set up your back foot just like an ollie, but with a little more heel hanging off, almost as if you were going to do a f/s 180, but not as drastic. Put your front foot just behind the bolts, with the ball of your foot resting on the heelside of your board. Your front foot is higher up than normal because you have to level out the ollie much quicker, so you can get a fast flip, because the board will not flip or will go out of control you are not leveled out or nearly leveled out.
Motion:
Set up and ollie as usual, but level it out as quickly as you can. Now, at the peak of your ollie, flick the board with your back foot, BUT, flick it in the center of the board, and as close to the heel side you can get without missing. The more of your foot you use to flick, the slower it will flick, you want just the very tip of your toe to flip it, and in the center of the board. Most people make the mistake of trying to flick it by the back bolts, and it doesn't work. the center of the board will give you the most level, and fastest flip, which is what you need. You also want to use your ankle, not your leg for the flip. Don't kick your leg straight down or you won't get it back up to land. Just get your foot to the middle, and while doing that tilt your foot upwards slightly using your ankle. This will give you leverage. Then, flick your ankle DOWN, and move your leg slightly BACK(the direction behind you would be, but only move it back a few inches). The moving of your leg back and ankle down lets you keep both feet over the board, but the moving of the leg back slightly helps the flick. Think of when you kickflip, your foot goes sideways, because straight down would make an uncontrollable flip. So moving your back leg back just a little helps for a smoother flip. So to recap the motion again, in short, ollie and level fast, back foot to the middle while raising your toes a little for leverage, then flick your ankle down at the very tip of the heel side of your board, and move your leg back just a little for the fluid flip.
Video Trick Tips:
Troubleshooting
Problem: I miss the board when i flick.
Solution: You are rushing everything. The hardest part is to take it slow, wait to get to the peak of your ollie, then flick it. Patience is the key to this trick.
Problem: I can't control the flip.
Solution: Wait to flick later, make sure you're only hitting the tip of your board at the center, and getting a good rolling motion of your ankle(toes pointed down towards the ground when you finish flicking).
Where Do I Go From Here?
Take it to whatever you want! Possibilities are endless. 180 late flips, late double flips, get creative!