The core plays a vital role in your ability to effectively change directions. Athletes are moving at high speeds and must stop, almost immediately, and move at a high speed in another direction. This is not an easy task. It is truly a skill that needs to be developed.
Have you ever seen a basketball player shuffling hard to the left, then have to go back to the right and get beat? Or an NFL defensive back running step for step with a wide receiver then lose the receiver in a cut? Changing directions is as much about the upper body as it is the lower body. What? How so?
Yes, the legs are the driving force when running. Yes, they take the brunt of decelerating. But what stops your shoulders from swaying (imagine a tree blowing in the wind) in the direction your body was just going? How you can run in the opposite direction if half of your body is still moving in the initial direction? Your core is the brakes for the upper body when cutting. If I am in that hard shuffle to the left and I change directions to the right, my core should engage and control my trunk. This puts me in an optimal position to get my body going in the new direction. It is the difference in cutting your man off in basketball, getting to the 50/50 ball in soccer, breaking up a pass in football or shaving a tenth of a second off your combine times. Developing strategies to teach the core to prevent or control movement is agility work and will take your multi-directional speed to a new level.