The core, midsection, torso (whatever you want to call it) has a front, a back, and 2-sides. Most people usually just train the front with sit-ups and crunches. That is ordinary training which will give you ordinary results.
For those who want true rotational power, (like when you hit forehands and backhands) you need to train the core in all directions.
This exercise, known as barbell russian twists or barbell rotations, trains your side muscles (like the obliques) in a similar movement pattern as when you hit forehands and backhands.
This exercise is #1 on my list for developing rotational power