During the act of fellatio, the
one who is sucking
will suck one of the sides of the suckee's penis up and down, thus side-shafting. Side shafting may lead to what is known as the 'Ultimatum of
Pleasure' which is the pre-ejaculation stage.
During a blowjob, the
female may or may not side-shaft
the man depending on how experienced she is or how good she wants the blowjob to be for the male.
Side-shafting can occur during a session of getting your
cock sucked well.