It functions as a third jump, but only if the computer controlled Nana is present. Popo throws Nana up a short distance while the two are attached by a rope, and Nana will then send Popo a long way upward by pulling on the rope, and both will enter the helpless state until they land, making them individually vulnerable until then. If the move is used too low, and no ledge is grabbed, Nana will likely fall to her doom while Popo will land on the stage safely.
Without Nana alive or near enough in the air, Popo merely does the motion for the belay, which will still grab the ledge when close enough, but gains very little altitude and leaves the Climber helpless to fall. Nana may also use the move alone to try and get back to Popo. Players typically use Squall Hammer when left with a single Climber.
In Super Smash Bros. Brawl and Super Smash Bros. Ultimate, Nana will aim for and grab the ledge of the stage provided they are facing toward the ledge, and will then pull up Popo, acting as a tether recovery. As both Climbers can now grab the ledge simultaneously, this effectively eliminates the problem of potentially being edge-hogged by one's own partner.
A technique using this move (known as Bungie Jumping) allows a player to edge-hog for an extended period of time with invincibility frames that can be repeatedly attained. The technique also sets up to chaingrab any opponents who get past the edge-hog.
In Super Smash Bros. Ultimate, if Nana does not make it back to the stage when the move is used, Popo can use the move again to rescue Nana if they are close enough.
This move seems to be unique to the Super Smash Bros. series, but it is possibly based on the Ice Climber's ability to "footstool jump" on each other in the two-player mode of Ice Climber. The animation is similar to when they jumped on each other's head while in midair in the original game.