I brought this off Amazon after my Freshman year of HS when the varisty coached told me my two handed jumpshot was holding me back. Practiced with it for a couple of weaks and it completely fixed my flunky guide hand.
That's the only thing that worked for me. And I tried a bunch of shit before that too. Squeezing my thumb to the side of my hand, crossing my index finger behind my other fingers, etc.
Not a seller or coach anymore but this product helped a ton with fixing the guide hand rotating for some of my past players.
I use these with my players. Well worth the price.