USMC boot camp
Susie represents your girlfriend when you're in boot camp.
Susie dutifully writes you letters and tells you all she does is think about you and hang out with Jodie. But really she's been with another guy.
Jodie is therefore really the other guy.
Then when you're doing leg lifts, a really tough exercise where you lie on your back and lift your legs up six inches for what seems like an eternity, the drill instructor shouts "Jodie!" and you put your legs together. He shouts "Susie!" then you spread your legs, to represent their respective positions when they fornicate behind your back. So while you're being tortured physically you're also reminded that your girl's probably banging another guy.