Hmm, If I buy at the lowest (12.18), and the price goes above the highest (13.46) what happens when the range changes to the new higher range?
Just to remind you of the basic idea: if you "buy", what you do is to promise to use a certain amount of bitcoin for buying $10 at 15.December 2012. You promise that now and are ready to place some BTC immediately as collateral. And you will be willing to take those bucks (the commodity you're buying) in December. But for the deal actually to happen, another participant in the market needs to say "ok, that price sounds OKish, I'll be selling 10 Dollars at 15.December for that rate. And so on.
You might be buying more future contracts, and your overall position will increase, you may sell future contracts, and your overall position will decrease, or even become negative (i.e. you turn from a dollar buyer to a dollar seller). All your various deals together make up an average rate (personally for you).
In your example, if you buy 1 contract at the lowest, you promise to buy $10 for 0.821 BTC (10 * 1/12.18). Thus, immediately 0.78 BTC +fee will be taken from your currency account, your position will be 1 contract and will be accounted at the rate 12.18
Now, to continue your example, lets assume the BTC and the futures market move up considerably. So, when the new session starts the current price will be at, say 13.60. Effectively this means, you will now need
fewer BTC to buy $10. To be precise, you only need 0.735 BTC now. Thus, your position will now be
marked to market: You get the difference of +0.086 BTC credited to your currency account as an immediate gain, and the effective rate of your position is now accounted as 13.60
Of course, you can any time close out your position by making a sell, and hopefully you're able to sell at a higher rate.....
