Each bit of Flag_CrabFollowing makes the item at the same position here follow the player: every time the player changes room, MovePlayerToRoom moves the item into the new room. Only bit 0 is used. ChangeRoom sets it when the player walks into The Rockpool while the giant crab is there, and closes the way out to the west and north; the crab then chases the player until they reach The Rocky Path or until the boulder falls on it (Action_Shout).