When guys are having sex with girls on bunk beds, when one guy gets tired of humping, the other takes over and his humping motion rocks the bunk beds so the other guy can keep it going with his girl while regaining his stamina.
guy 1: "Yo bro bunk bed sex
is the best cuz when i get tired, you can fuck
your girl and rock the beds for me so i can take a break while still getting off
guy 2: "What!?!?"
guy 3: "Yea man you gotta ride the train at least once in your life"
guy 1: "thank you!!! thats what im talking about!!"