For the Mason Twist, sit comfortably on the ground with your hands clasped together in your lap. Next, while slightly rocking your torso back, lift your legs off of the ground to that your weight is supported just under your tail bone. From this starting position, rotate your torso till your clasped hands hit the floor. Continue by rotating all the way to the other side and, again having your clasped hands hit the floor. Repeat this motion for the allotted amount of reps and then rest.
This exercise focuses on your core. The oblique muscles on your sides are heavily activated when rotating your torso in the crunched position. Add weight in your hands to make the exercise more difficult.