A killer location you can do even if you are blind. I looks like if you can find any card by the scent of the spectator. She only has to blow to any card she picks with any borrowed deck! 100% impromptu.

She can shuffle before and after, and as much as she wants. You never touch or see the chosen card. You can be blindfolded or in another room. When you back you will find her card in less than a second, even with your eyes closed all the time!
No forces of any kind, no angles
No gimmicks, no marks, no tape
Nothing extra but a regular deck
No peeks, no fishing, no due reality
Easy, no order, no memorization
No maths principles, no luck
No by smelling, no help from anyone

Actually no need the spectator blows to the card, it is just presentation.
Same for a single person and for a big stage theatre.
"Wow, I can't believe this is actually possible!"- Gaetan Bloom