String keeps falling off thumbs and yoyo finger
When your making the wrist movement to make that slack, try to give it a more downward motion. also, before trying to actually do the trick just work on getting that slack to form perfect, then put the whole thing together.
Keep your thumb pointed parallel to the ground or upward so the string will be less likely to fall off. Also, the string doesn’t leave your thumb until you start moving your throw hand down, so forming the slack is all in the downward motion.
Also, don’t swing too hard. That was the mistake I was making when I first started. Try to hop the yoyo just above the bottom of the slack loop that forms. Makes landing it on much easier and with a bit of weight on the strings, it’s less likely to slip off your thumb.
Point your thumb and finger up to keep it from falling off
Practice it with the yoyo dead and just holding the yoyo and getting a feel for the whip. It can really help.