The number of right descents of a signed permutations.
An index is a right descent if it is a left descent of the inverse signed permutation.

The Kreweras complement of a signed permutation.
This is the signed permutation $\pi^{-1}c$ where $c = (1,\ldots,n,-1,-2,\dots,-n)$ is the long cycle.
The order of the Kreweras complement on signed permutations of $\{\pm 1,\dots, \pm n\}$ is $2n$.

The signed permutation with all signs positive.

This map sends a permutation $\pi$ to $\pi^{-1} \star \pi$ where $\star$ denotes the Demazure product on permutations.
This map is a surjection onto the set of involutions, i.e., the set of permutations $\pi$ for which $\pi = \pi^{-1}$.