Riku uses Helm Split during the "Riku (Keyblade)" battle. It consists of him leaping into the air and plunging the Keyblade of People's Hearts into the ground a single time to hit Sora with several columns of dark flame.
Kingdom Hearts Re:Chain of Memories
The Riku Replica uses Helm Split as one of his unique Sleights, starting with the second battle against Sora, and both battles against Riku. After Riku Replica strikes his target from above, he concludes the Sleight by hitting Sora or Riku from below with several columns of dark flame.
Kingdom Hearts 358/2 Days
Riku uses Helm Split during his battle with Roxas at The World That Never Was. This version includes teleportation, which allows Riku to follow Roxas around the battlefield while executing his downward aerial strikes. Riku will appear directly above Roxas and slam down three times, each attack creating a shockwave upon impact.