And so your gluteus maximus fires just before your body's about to—your trunk is about to pitch forward and make you hit your nose on the ground, and it helps pull your trunk backward.
And the other time the gluteus maximus fires is when your leg is swinging forward when you're in the air, and it helps decelerate the leg so that you bring your leg down onto the ground.
And it turns out that very nicely we can see when the gluteus maximus got big in human evolution because its upper portion, the portion that's really important for this function, leaves a trace on the pelvis, on the bone.