The transverse abdominis is a deep core muscle that acts as a natural weight belt for your spine. It's the Spanx of your abs. Developing it will help prevent injuries and create a smaller waist. You could train it with plank variations, but I like this exercise using an ab wheel and a treadmill.
Hold the ab wheel the same way you would for a rollout, but instead of rolling out, you hold it like a plank. This will also hit the internal obliques. It's fun and quite possibly a better use of the treadmill than jogging for abs. The slower the treadmill goes, the harder it is.