In short, once one of your friends is done or broken up with a girl, you silently move in to support the girl in her time of dire need. Crushed and heartbroken, the girl will talk incessantly, but will eventually open up sexually. It can be noted that this system requires almost minimal effort and simply nodding and listening works, combined with a few hugs.
The M.A.C. System was made popular by Mac, a fictional character from It's Always Sunny in Philadelphia. His system is highly underrated and deserves as much attention as the D.E.N.N.I.S. System.
Dennis: "What is this swooping in business?"
Mac: "Oh that's my system,The MAC: Move in After Completion. I wait til you're done with them and then I swoop in, give them a shoulder to cry on, then we hump."
Dennis: "You've been humping these girls after I'm done with them?"
Mac: "Oh yeah dude! I'm swimming in your wake."
Robert: "Hey, Charlie, you get laid this weekend?"
Charlie: "Yeah, Mike broke up with Holly on Thursday so I used The M.A.C. System on her and we made sweet, passionate love."
Robert: "No kidding? Bump it."