Ah, a how to I can help with......
You have to push harder. When you're rolling backwards with one foot on front and the other on the back peg, just shift all your weight to the back, pull the bars really hard, like 2x as hard as you think you'd have to, into your chest and then push them out in front of you. All you're weight will automatically go to the back peg, and you'll coast around. Super fun trick. The balance is easier than a hang 5, when you are in the trick it feels like a regular peg wheelie.