I position it with my legs straight and then as I'm going up the lift, I relax a bit and sorta slouch. Once the drop comes, I stand straight up...
When I position the seat, I have the restraints firm on the top of my shoulder blades which leaves room for down below between the bicycle seat. For me, my head is perfectly in place for the "toilet seat" top as I like to call it and having my head sorta pressed into that restricts any sort of head banging. That's not needed throughout the ride, mainly entering the final helix right before the final bunny hops/ brakes.
With my shoulder blades being firm against the top of the restraint bar, it restricts my body from moving around. Just don't squeeze the restraints inward too much, otherwise it pinches your ribs/ sides.