What You'll Need
A mat... or simply a floor. Your call.
Why You Should Be Doing This Exercise
Sometimes the beauty is in the simplicity. This requires you to move 5 inches in either direction, but you do what needs to be done here, and that'll prove to be a journey worth taking. Strong core requires strong obliques. Strong obliques require good exercises, like this one.
Lie down with your back on that mat or the ground. Bend your knees, and spread your feet some. Cool. Now make sure your arms are straight, and by your sides. Cool, you're ready to begin.
All that needs to happen here is the fingers on your right hand need to touch your right heel. Once that happens, it's left fingers to left heel. So, how should those touches happen? By crunching forward and sideways a bit with your upper back off the ground. Because just going forward won't allow the hand to hit the heel. It's the sideways movement that'll get those elusive obliques.
One tap to one heel is a half rep, so don't think a set of twenty here is ten taps to each heel. Ha, silly rabbit, tricks are for kids.
Keep your back flat, and keep your neck nice and relaxed. No need to tense up, you'll be just fine.