A covert way to tell a person to screw them self, or a way to insinuate that a man is literally screwing them self. 270 is the angle of the penis necessary to perform such maneuver, hence the brief and scientifically perfect synonym for an otherwise vulgar gesture. It is also a great way to respond to a person who is bragging about them self.
This number is used in the “World Fame” marching band of Grambling State University to depict a movement that resembles a Right-Face with a little spice to it.