So say I'm selling a contract for 1BTC for $700, instrument 6.14. So if I'm correct, on June 14, 2014, the buyer of my contract will not pay me USD (since everything is in BTC), but in BTC.
When you sell 1 contract, you are not selling 1 BTC, you are selling $10 of BTC (at the price specified).
So if you want to create a position equivalent to -1 BTC, and the current trading price is 700, you need to sell 70 contracts (70 x 10 = 700, give or take a little for fees etc).
You then have a floating position of $700 worth of bitcoin, against which you will accumulate profit or loss, if the price halves and $700 buys twice as much bitcoin at expiry, then you will pocket the difference (in bitcoin).
You will see yet net value of your position fluctuate on a daily basis as the ICBIT price for that instrument fluctuates. If you hold to contract expiry you will see the ICBIT price converge with the settlement spot price (e.g. Bitstamp) in the weeks/days before settlement.
Make sense? If not the best way to learn is just buy sell 1 or 2 contracts and see what happens, you'll soon get a feel for it.