The Shunt is a piece of climbing equipment made by Petzel. It is a medium sized ascender. It allows one way movement up a rope. While your weight is on the shunt it lock onto the rope. When you take your weight off the shunt you can move it along the rope. Usinf a combination of shunts you can load them alternativly and hence shimmy up the rope.

The traditional method for doing this involved tying prussock knots onto the rope. The shunt is more reliable and easier to use. The drawback is that it is quite a bit heavier than carrying a few prussock loops with you.