If Winklevoss twin exchange all their BTC...

[hacker1001101001]

I don't think it will affect much as Winklevoss twins own bitcoins worth 1 Billion market price now and the market cap of bitcoin is approx 130 billion. SO, as compared to the total bitcoin is circulation they don't own the amount needed to manipulate the market. Bitcoin has also experienced bigger frauds and hackers than this but it is still a very stable currency in the market.

[shinchan123]

BTC’ course go down?

The bitcoin price will crash down some more if that happens, however, I doubt if they will be dumping all their bitcoin all at once since they do not have a serious reason at all. They already have many businesses and as a businessmen I know that they are aware that it will be a foolish thing to do.