Commute

is a monadic operator that takes a dyadic function as operand and modifies how the argument(s) of its derived functions are used. If the derived function is used monadically, the argument is also used as left argument. This usage is also known as Self or, more informally, Selfie. If the derived function is used dyadically, the arguments are swapped. This usage is also known as Swap. The glyph is also used for Constant.

Examples
This decrements from the vector:

Double:

Commute can be used to emulate a monadic  Fork when combined with Compose (note the following code is not workable APL, but a series of evaluations):

Lessons

 * APL Cultivation

Documentation

 * Dyalog