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.
One of my central operating paradigms is the realization that all methods, devices, philosophies and techniques involved in strength training have specific benefits and drawbacks. If your training lacks sufficient diversity, you'll accumulate the drawbacks and habituate to the benefits. And that ain't good.
Most of you just have an ab, as in one non-distinct blobby area between your pecs and your naughty parts. Here's how to make your singular ab into plural abs.