Many customers use Bitcoin as their margin to trade, so we have Bitcoins to sell when there're short positions.
Are you referring to having BTC to sell on Mt. Gox so that you have MTGUSD available for when those shorts cover?
But that is an um, ... unique approach. With each 1.0 BTC giving $20 USD (at currency BTC/USD rate of $4) of buying power, this would help explain why the concept we know as the weekend dip may be nearing a conclusion.