Begin in a high plank position, with your hands directly under your shoulders and your legs extended behind you. Keep your core braced and your head and spine in a neutral position. This is your start position. Pushing against your hands and bending at the knees, move your torso back until your glutes are level with your feet. Quickly reverse the movement and return to the high plank position. Repeat this back and forth motion for the required number of reps or time.