This exercise can be performed with your chair pushed against a wall for extra stability, or free-standing for a greater challenge.
Place your hands on the center of your chair's seat, pressing straight down through your palms. Make sure you're not pressing forward, which would cause the chair to roll. With your shoulders directly over your palms, step your legs backward, planting the balls of your feet on the ground so your body forms a straight line from head to heels. Hold the position for the duration of the exercise.
You can make the move harder by engaging the swivel on your chair and rotating your palms to the right and the left as you hold the plank.
This exercise actively uses the rollers on the chair, so make sure you have some room to move.
Start on your hands and knees in front of your chair, but facing away from your chair. One at a time, lift each leg from the floor, placing the top of each foot onto the chair's seat. Extend your legs so you're in a high-plank position, your body forming a straight line from heels to head.
Now engage your abs and press down through the tops of your feet as you bend your knees, drawing them toward your chest. The rollers on the chair will cause the chair to roll toward your body. Once you've pulled your knees as close to your chest as you can, reverse the movement, extending your legs as you roll the chair away.