Split stance plus band, glutes on fire.
Keep the band tension constant and your torso tall, don't tip toward the front knee.
Elevating your back foot and squatting on one leg is already one of the nastiest quad builders around, and running a band from under the front foot to your shoulders adds resistance that grows the harder you drive up. Each leg works alone, the front quad handling the lift while the glute keeps the pelvis square over a narrow base. The band's pull also punishes any forward tip of the torso, so it quietly coaches the tall, stacked position that makes split squats effective.
Under the arch of the front foot, with the ends held at your shoulders. Anchoring under the front foot keeps the tension tracking straight up through the working leg instead of dragging you backward.
Usually the stance is too narrow, like standing on a tightrope. Set your feet on two rails hip-width apart, keep your eyes on a fixed spot, and lower straight down rather than forward.
Both are working, but a vertical shin and upright torso bias the quads, while a longer stance and slight forward lean shifts more to the glutes. This banded version with the tall torso leans quad-first.
