The send party Pokémon to a new game glitch is a derivative of the SRAM glitch in Pokémon Red, Blue, and Yellow.
If the player performs the SRAM glitch (go to save on a new file but reset shortly after the Yes/No box disappears while saving) with an existing save file, the party Pokémon from the old save file will be transferred to the new game, but unfortunately the Pokédex won't be updated, nor will the player have their old items or the expanded items pack.
Additionally, the player will lose all badges. They will still have '0' Pokémon in the Pokédex until they obtain a starter. High leveled Pokémon may not obey the player due to them still having their old ID number (and OT name if the player uses a different name for the new save).
There is a positive side for the Pokémon not getting registered. This prevents the Oak's Parcel prevented progress glitch from happening in Pokémon Yellow, unless the player evolves one of the Pokémon they transferred over to the new save.
This glitch is particularly useful for getting HM moves early if one or more of the party Pokémon had a HM move.