The partial escape instant victory glitch, also known as CoolTrainer-less instant victory glitch is a glitch in Pokémon Red, Blue, and Yellow. It allows for the player to activate a form of instant victory.
Summary
When you use 9F (Red/Blue, hex:5E) or -g m (Yellow, hex:6A) in a wild Pokémon battle, you are able to partially escape the battle. However, after using a Master Ball, a glitch will be activated with the next Trainer battle or wild battle in which using a turn using item (Poké Flute and Potion if your Pokémon doesn't have full HP confirmed) will instantly end the battle.The effects do not persist for battling another Trainer afterwards, so in that way it is 'single-use' until you battle another non-D059/8 wild Pokémon (no Trainer escape Pokémon or hooked Pokémon).
Wild battles are normally disabled after using the partial escape item, but are re-enabled again after flying away. Furthermore, the glitch can still be used after using the Master Ball and flying away if you haven't done it yet, allowing us to do things like beat Gym Leaders and Lorelei of the Elite Four.
You can get 9F or -g m with the Celadon looping map trick from dry underflow glitch.