This probably looks like a fairly complicated trick. Don't worry though, it's really not. There are just a few important things to remember:
1. Make sure your freehand index is intercepting both parts of the loop of string, not just the one closest to you.
2. Make sure your freehand index is right next to the left side of the yoyo as the loop starts swinging around your index. Keeping your hand close to the yoyo will make hitting the axle with the far side of the loop easier.Keep those in mind, and you'll do more than fine.