A short-lived counterdirectional wave or surge, usually produced as a dying line of whitewater rushes up a canted beach, turns, and flows back into the surf zone. The resulting backwash—which can appear as a small unbroken swell or a small foam- fringed wave—can then ruffle or explode across the next incoming wave.

Surfers at Makaha, on Oahu's west side, have for decades made a game of facing off against the break's famous backwash, which is often big and strong enough to itself be ridden. B...