From Glitch City Laboratories
The draining hitpoints animation glitch is a minor glitch which only exists in Pokémon Ruby and Sapphire because of an improved battle system on later handheld games. If the player's Pokémon deals a very large percentage of damage on a wild Pokémon the Hitpoints animation of that Pokémon will not drain but rather its Hitpoints will appear to instantly drop to zero, as if the opponent had used Explosion.
- A battle situation which is very beneficial for the player, such as a Level 100 Kyogre with six Calm Mind boosts that is about to use Water Spout.
Method one (with a Kyogre)
- Encounter a relatively low level Pokémon, ideally one which the player can get a Super Effective hit on, such as Sandshrew.
- Send out Kyogre and make sure that Rain has activated and the opponent does not have either Air Lock or Cloud Nine.
- Get six Calm Mind boosts on Kyogre and ensure that the player has not taken much damage.
- Use Water Spout and if the percentage damage dealt is over a particular value the opponent's hitpoints will instantly drop to zero, with no hitpoints drain.
-  - A list of minor glitches from Pokeglitch.tripod.com, mentioning this glitch.
-  - A page from an article on Serebii.net forums mentioning the glitch.