⍨) 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.
This decrements from the vector:
1 -⍨ 1 2 3 0 1 2
+⍨1 2 3 2 4 6