This move is a great way to tone — you guessed it — the triceps, which are the muscles that run along the backs of your arms (the ones that tend to get jiggly when you've taken a break from exercising).
Here are two ways to do it.
For beginners: Stand with your feet hip-width apart. Lean forward slightly (hinge from the hips — your back should stay straight), keeping a slight bend in your knees. Pull your elbows in close to your torso. Extend your arms behind you, with your palms facing each other. Squeeze the triceps at the top of the extension. Keep your core tight and your shoulders rolled down away from your ears. Return your hands to your sides, and repeat for a total of 30 reps.
Make it more challenging: Grab one dumbbell. Hold the weight in your left hand and step your left leg back into a wide lunge, with your right knee bent at about a 45-degree angle. Rest your right hand on your right thigh and hinge forward slightly from the hips, keeping your back straight. Pull your left elbow in close to your torso and extend the weight behind you; squeeze your tricep at the top of the extension. Return the weight to your side and repeat. Do 30 reps on each side.