Rundown is the jizz that runs, or flows down the shaft of an erected penis upon ejaculation. It refers to all jizz that makes its way down the shaft, whether from sex with a tight vagina, or from masterbation.
After Mike ejaculated from Michelle's hand-job, the rundown came flowing down his penis.
A really anoyying but kinda fun at the same time flash vid with a muscularly undefined guy lipsinking the words and having a good time with it. Be careful it can get stuck in the head almost as easily as the Hampster Dance. If you really want to find out more or want to see it go to www.newground.com, it's where I found it. Either it will be on the front page or in the portal.
It's like the hampster dance but with an actual person and no toons.