Setting for a back tuck can be a tricky concept to grasp when first learning the skill, but it is imperative to progress later on to a layout and eventually a full.
I would be willing to bet that your difficulty setting is due in part to your back handspring. In my experience, a lot of people who whip their head back in a tuck to attempt to rotate because they come out of their BHS with their chest down.
Remember, your upper body should not be the primary force of rotation, but instead should act to gain height while simultaneously allowing the body to rotate backwards. This happens when you punch the ground out of a BHS with your chest tall. squeezing your core tight, and keeping an "open shoulder angle(arms fully extended by, or slightly behind, your ears.)" Your lower body should initiate your rotation as you begin to tuck and drive your shins, knees, and toes over your head. (much like the rotation of a backwards roll.)
In other words, focus on "standing up" out of your back handspring. Really snap hard and concentrate on squeezing your core, keeping your head in, and riding your tuck up like you would a rebound. At the top, be aggressive when tucking because it might feel weird the first few times you rotate correctly. It might even feel slower, or like you are beginning to stall since you are used to your head leading the rotation, which used the bounce from the floor to rotate right away.
Have a trained instructor spot you the first few times so you can focus on the correction and not be afraid to make the changes out of fear of injury. If you learn visually, a visual cue like watching the wall might help you keep your head in like "cheerforeverever" said earlier. Or if your prefer have a coach shout a verbal cue like "squeeze" or "stand up" to remind you while your going for it.
Hope this helps,
Coach Matt
