It's very similar to a company spinning off a subsidiary, or paying a large dividend.
The next day after it goes ex-div (or after the spin-off), the parent company shares are trading at the previous price minus the value of the div/spin-off.
Based on current market pricing, if 1BTC=$2750 and 1BCC=$400, then immediately after the fork it will be 1BTC=$2350 and 1BCC=$400. Then the 2 coins will fluctuate independently (they already do implicitly in the futures market)
Is that how this works though? How is the value determined when it forks. The futures market has such little volume, it doesn't seem like a reliable indicator of where BTC might be when it forks.