I'm all about finding new ways to work the abs, which is one reason this exercise has become one of my favorites. The figure 8 involves passing the kettlebell from hand to hand behind the legs as you make a figure 8 motion. This move works on activating the stabilizer muscles of the core while also challenging your balance and coordination.
I love this move as a warm up before a dynamic ab workout or just as a way to make my clients stare at me in confusion. It does require concentration and you'll want to start slowly to avoid dropping the weight (which does tend to happen, so do this move on a mat to be safe).
Do it right: Hold a medium kettlebell in the right hand (I used a 10-lb weight here) with feet hip-width apart and lower into a squat. Bring the weight between the legs, grabbing onto the handle with the left hand behind the left leg. Circle the weight around the front of the leg (you can add a swing for intensity, as shown in the picture, or just keep it small), again bringing it between the legs and grabbing onto it with the right hand behind the right leg. Continue moving the weight in a figure 8, exchanging it from hand to hand, for 1-3 sets of 8-16 reps.