Horray for glitches!
This weird error surfaced in rijonAdventures and I need to get it fixed. But yeah, that’s a known glitch for you all to know. 😛
To multi-select Pokémon in a box, you press the SELECT button to change the cursor into quick select mode. You then press and hold the A button over the Pokémon you want to select. Normally, the Pokémon highlight in red, but here they disappear!
This feature exists in FireRed, LeafGreen, Emerald, Diamond, Pearl and Platinum. You will probably also have spotted it in Pokémon Box.
Originally posted on the old Rijon website, now offline. Archive link.