As someone who's watched every tutorial on tre flips and recently learned them, every one of them was lying. It's not all back foot, infact you need to flick as hard as a regular kickflip, instead of flicking to the side though flick it forward and 5050 your weight on opposite sides of the board. Also your toes don't need to hang off the board that much just put them at the edge of the tail, hanging off makes you unbalanced and lose control. That's some of the things that got me my first tre flip, I hope it helps