These are rapid back-and-forth shuttles with one step in between direction changes. Go two steps to the right, then two to the left.
This really focuses on lateral change of direction, and it's a great way to improve coordination and leg strength/power while building structural integrity in the lower body. Higher reps will scorch your quads and lungs.
A bar takes the ab rollout to the next level. Try a wide grip to hit the chest and shoulders as well as the abs. Have a super strong core? Narrow the grip.