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.