Drive the knee up against the band to fire your hip flexors.
Keep your hips level and your standing leg tall, don't lean back to cheat the lift.
Driving a knee all the way up toward your chest against a band looped around both feet takes the hip flexors and the rectus femoris, the quad that crosses the hip, through their full range under load. The higher the knee climbs, the more the band stretches, so the top of the drive is the hardest part, exactly where most people's hip flexors are weakest. Meanwhile the standing side's glutes grind away keeping the pelvis level, one leg working, one leg stabilising.
Range. Driving to the chest instead of hip height loads the hip flexors in their shortest, weakest range, which is where they usually fail, it is the fuller, harder expression of the same movement.
Leaning back shortens the distance the band travels and cheats the top of the rep. Stay tall against a wall, keep the standing leg long, and let the knee climb only as high as your posture allows.
Strong hip flexion is knee drive, and knee drive is stride power. If your knees stop lifting late in runs or sprints, this is one of the most direct ways to train the muscles responsible.
