Try to keep the legs in the same spot. Doing this and making the movement more of a crouch will make it look more like he's bracing himself for the launch!
Also, tweak the shape of the Snap-back, try and have it look more loose right after snapping, it'll give it a more rubber-bandy look and make it easier to follow the snap!
Elastic materials like rubber bands (or, well, ELASTIC) tend to twist a bit as they lose tension rather than just flinging straight up like a sling would.\
You can sorta see what I mean in this gif here:
EDIT: found a better example