The single-leg builder that humbles everyone.
Keep your weight on the front heel and drop straight down, not forward.
Elevating the rear foot loads almost everything onto the front leg, so the quads and glutes on that side work through a deep range with half the weight a barbell squat would need. Descending straight down keeps the front shin and thigh doing the work while the trailing leg just balances. Trained one leg at a time, it also exposes and fixes side-to-side strength gaps, which is why it has become a staple leg builder rather than an accessory afterthought.
One leg is handling nearly all the load through a big range of motion, plus balancing at the same time. The deep stretch on the front leg is exactly why they build so much muscle, the discomfort is the price of admission.
Far enough that you can drop straight down without your front heel lifting. If your knee slams past your toes and your heel pops up, step further out. If you feel pinching in your rear hip, shorten it slightly.
Both, and you can steer it. A more upright torso with a moderate stance biases the quads, while leaning your chest slightly forward and taking a longer stance shifts the emphasis into the glutes.
