It's the equivalent of a hacker's secret handshake.

To ease stiff shoulders, do a 'secret handshake' stretch - with one thumb up and one down, clasp hands and pull.

Is it a secret handshake?

The tell-tale sign will be when he starts wearing a little red stud in his buttonhole, the legionnaires' version of a secret handshake.

Usually a secret handshake has underlying meanings that differ from person to person.