When you are in great need of a poo of epic proprtions. Said poo will be so destructive to the toilet that the carnagewill be similar to that inflicted on the german town of dresden by allied bombers during world war 2. Therefore your bomb bay is full and you are 'approaching dresden.'
Guy 1: ''Why do you look so uncomfortable''
Guy 2: ''I'm busting for a crap. Seriously, it feels like I'm approaching dresden here.''