It is similar to the old man trick but makes use of an in-game trade instead of the old man's demonstration. Furthermore, according to Okk's Missingno. guide, this was the first version of the old man trick discovered.
Summary
This is a glitch where the player performs an in-game trade, and then surfs on the coast to encounter level 80 Missingno. x5 (and up to five other Pokémon the player might have previously encountered in the grass before trading).This works because the name of the trade NPC is written in the grass encounter rate address and the addresses that follow (level and species addresses). The specific data is 5D 50 50 50 50 50 50 50 50 50 50 (5D is a control character that prints "TRAINER" and it is followed by 10 'end name' characters). The 5D (grass encounter rate) is irrelevant because the water encounter rate is used and Cinnabar Island resets the grass encounter rate to 00.
Since characters 2-11 control level and species (with even characters determining level, odd characters determining species) the player reads 0x50 (80) as levels and 0x50 (Missingno.) as species.