For every 5 shares of the COIN ETF the winklevoss brothers will hold 1 bitcoin. If there is demand over the amount of coins they already have, the will then buy 1 bitcoin from the market for every new 5 share purchases so if it takes off it can create some serious demand for bitcoins.
That's not how it works. COIN doesn't need to create and destroy shares or otherwise manage the price. There are some accredited entities who will act as arbitrageurs. That is, if the price is different for the ETF on Nasdaq vs. Bitcoin at Bitstamp, they will buy at one place and sell at the other until the prices adjust just as they do today across Bitcoin exchanges.