Done by visiting the exterior of a cave with a partner. Splitting from the partner, then returning to the cave's original entrance and using Escape Rope brings the player back to the exterior (not the entrance).
From there, going back into the cave will join the player with the partner, though not from the intended point (i.e. at the entrance). Then, the partner will follow the player in strange patterns - similarly to how Pikachu followed a player in Yellow's Glitch City or during the Hooked Dragonite trick.
Any practical use for this? I haven't played B/W or B2/W2, so I'm not familiar with any new game concepts that could be used in conjunction with this quirk.