Get on the floor on hands and knees. Keep back straight as with the plank.
Keeping back flat and still, squeeze the glute to move one leg back and slightly to the side (like a donkey kicking).
Note: the movement should be initiated from the glute (butt), not the lower back. If you feel the lower back working instead, start with smaller movements until you can increase the range of extension using only the glute.
Perform 1-5 sets of 8-12 reps per leg. Or, perform 1-5 sets of 20-40 second intervals (doing as many reps as possible during those intervals).