you open the door with the breasts and go in. the room is dark, save for a solitary light that illuminates you. suddenly a tingling sensation fills your body, rather like a low dose of electricity. then your waist starts to shrink. it shrinks so much that your pants fall off. your hips swell, becoming large enough to catch your decending boxers. your butt grows large and generates a massive rip in your underpants. the hair on all of your body save the top of your head falls out. your face becomes feminine, you can feel it with your now feminine hands. the changes accelerate, and your pecs bulge, turning into massive breasts (as the sign indicates)tearing your shirt to ribbons. your hair grows extremely long. you have transformed into a sexy bimbo!
(the exact details of your transformation are up to your imagination).