Top Tuck = within a thin corridor, climb your way up to the ceiling using wedge and brace methods. When you are lodged in position with feet on one wall and lower back against the other, slowly bend forwards and shift your way up the wall so that your head touches the ceiling. Then carefully manoeuvre your arms under your knees.
Caution - this final stage of the move can be dangerous as the bending forwards can loosen your wedged position.