This is one of my favourite exercises! It works predominantly your core muscles but you also need to engage your legs to lift them up off the ground and your arms also to hold a dumbbell or kettlebell up in front of you.
Sit on the floor or a mat with your legs stretched out in front of you. Pick up the dumbbell / kettlebell and hold it above your waist while in the seated position. Raise your feet about 6 inches off the floor and proceed to perform a twisting motion with your upper body so you end up touching the ground either side you with the weight. Perform 20-30 reps about 2-3 times with a 30 second rest in between.