When you are having sex with someone in any position that has you behind your partner, ( doggystyle
, etc... ) Before you engage in intercourse, Face your partner to a window then start having sex in one of the previously mentioned positions. Then, tell her you need to use the bathroom really quick and not to move from her position so she will be ready when you get back, but instead have your friend come in and start having sex with her in previously said position. Then walk yourself outside in front of the window and start waving.
''Hey dude you wanna pull the window man on this chick i just met tonight at the club?''
''Hell yeah bro''