Sweep behind, load the glutes, stay graceful.
Keep your hips facing forward as you cross behind, don't let them twist open.
Stepping back and across behind your standing leg drops you into a lunge with a lateral twist to it, and the quads of the front leg drive the whole climb back up. The glutes work overtime here because the crossed-behind stance loads the hip from an angle straight-line lunges never touch. It fills the gap in leg training for anyone whose lower body work all happens in one plane.
The crossed stance is unusual, not dangerous, but it does demand control. Keep the step behind modest, drive through the front heel, and shorten the range if the front knee wobbles inward. Build the movement before you build the weight.
Both, for different reasons. The reverse lunge is the straight-ahead strength builder, the curtsy adds the lateral hip angle that standard lunges miss. If your legs only ever move forward and back, the curtsy is the one you are missing.
You are reaching too far behind. Shorten the crossover until your hips can stay square to the front, then lengthen it gradually. Facing hips forward is the whole point of the setup, so protect it.
