To clarify, there is a teenage mutant ninja turtle - a children's cartoon, and then there's "the ninja turtle". The ninja turtle is the act of taking your ballsack testicles included
and flipping it around so that it sits on top of your johnson. It's best achieved on a warm day when things are hanging lose. The term originated at the University of MD fraternity scene. The ballsack resembles a turtle shell and the johnson is the turtle's head peeking out.
Jim was so drunk that he pulled down his pants and demonstrated the ninja turtle in an attempt to get Cindy's attention. But he failed miserably and went home and pulled his pud instead.