Is a joint rotation system.
While 2+ people are sharing a joint, one of the participants may call tango. In that case each one of the participants takes 2 hits and passes the joint to the next participant (clockwise for us old-fashioners).
Derived from the tango dance. Since it takes two to tango.
It's also thought to be the most efficent method to rotate a joint.
Let's make it friendly.. let's tango