Climbing

A zipper is a series of forcing moves by which a player gains significant distance across the board and potentially connects to the other side.

Here is an idealized example. Red to move.  Red wins by zippering from e8.  Note that every single one of Blue's moves is forced. Although Blue could intrude into some of Red's bridges or other templates, this does not help.

Note that the zipper was possible even though Blue seemed to have more strength on the right side of the board than Red. What enables the zipper is the exposed "flank" of unprotected Red pieces. Potential zippers are often difficult for beginners to spot, and can lead to swift and unexpected defeat. It is therefore a good idea to try to deny the opponent opportunities to zipper.

Example
Zipper do not always have to proceed by bridges, although a combination of bridges and adjacent moves is the most common. Here is an example from an actual game. Red to move.  Red starts a 3rd row ladder once, then immediately jumps and zippers.  At this point, Red is connected to edge template IV2d by an interior ziggurat, marked with "*". Blue resigns.