Stand over a tire or a heavy target with your feet wider than your hips, sledgehammer held in both hands. Slide your top hand down to meet the bottom one as you lift the head of the hammer overhead, then drive it down by rotating through your torso and hips rather than just your arms, like chopping wood. Let the swing finish across your body and follow through low, then use the rebound off the tire to help carry the hammer back up for the next side. Your core does the real work, bracing against the twist and absorbing the impact through your midsection each time the head strikes. Keep your knees soft and your grip relaxed until the moment of contact so your wrists aren't taking the shock of the hit.