You do it to a confederate who is planted in the audience. She is paid to make believe it is her bra, and to react as embarrassedly, or provacatively sexily, or whatever, as you and she decide in advance.
That's probably the worst advice I've heard in years.
Surely this falls along a similar effect of Scarne's (and others') "Shirt Off" where you unbutton a couple buttons of your spec's shirt, and pull the shirt off him by yanking on the collar.
Very popular effect in it's time.
And thinking about this effect, for the right audience, why couldn't you do this with with your female assistant? Or "spec".
And you know, every time I see a woman with a pink thong riding up her rear, I think of performing 20th Cenury Panties. Surely someone who loves the attention of wearing one in public, would have no ill thoughts when you magically remove it.