If you hold a call, when you exercise it you will get BTC in exchange for USD, yes.
Exercising a put gets you USD for your BTC when exercised.


So, you buy a put, perhaps it's at a strike price of $10. You pay US $3 for each put, and buy 100 of them.

You need $300 US in your account; that goes over to the put writer immediately.
The put writer needs $1000 USD in her account; that is locked immediately, and the exchange will hold it until expiration.

Let's say BTC tanks to $5 per. Bummer for everyone else, but not for you! You can now exercise the put and give 100 BTC to the writer in exchange for her $1,000.

You netted $200 in this case.

The put writer traded $700 in the end for 100 BTC.

Does that help?