Template:MoveInfoGenI
TM21 is a (variable name) (0x28)-type glitch move in Pokémon Red and Blue. It is not capable of causing Super Glitch.
Successfully using this glitch move may deal damage with Hydro Pump's animation, and then either partially end the battle, allowing the player to open the Start menu and move around (if in a normal wild Pokémon battle), or restart the battle (if in a scripted wild Pokémon battle or Trainer battle) if it does not knock out the opponent.