Back foot up, front leg does it all.
Keep your weight on the front heel and torso tall, don't push off the back toes.
Elevating the rear foot strands nearly all your bodyweight on the front leg, so the quad grinds through a full squat range on its own while the glute controls the hip and drives you back up. The stretch on the trailing hip at the bottom is a free bonus most exercises never provide. It earns its reputation because a bench and your own bodyweight are enough to humble legs that back squat respectable numbers.
Because one leg is doing what two normally share, through a full range, while also balancing you. The rear-foot elevation removes the back leg as a helper almost completely, which is exactly why the exercise works so well.
It can drift forward somewhat, especially if you are targeting the quads, as long as the front heel stays planted and nothing hurts. Dropping straight down with a tall torso, like the setup describes, keeps the knee tracking well by default.
First add reps and slow the lowering phase, then pause at the bottom. When that stops being hard, holding weight in your hands is the natural next step, but a strict, deep, paused bodyweight rep takes longer to outgrow than most people expect.
