Most abdominal exercises do not work the transverse abdominus, since it is an internal muscle with fibers that run horizontally. Why work it if you can’t see it? Because it gives your torso stability, making other exercises more efficient and helping prevent injury. It also holds your gut in, essentially flattening your stomach. The transverse abdominus aids in forced expiration, which means you work it every time you breathe out forcefully while completing squats, deadlifts, and other heavy exercises.
A good way to work your transversus is by doing a simple movement called a “vacuum” exercise. To do this, exhale forcefully, suck in your gut (contracting it so that it’s smaller), and hold it for a couple of seconds. You may do this exercise as much as you like, as there is very little chance of incurring an overtraining effect from it.