EDIT: I'm not sure what the above poster meant by "about to cash out." Filing or approval in no way means Winklevii will be selling any or all of 100k coins at market the instant they are approved. They have to own the coins that the shares represent. They don't get to just sell them when ETF is approved, that makes no sense and then they wouldn't have an ETF. They hold the coins as specified in the filling. As more investors pile in they'd have to purchase more coins. Or the opposite. You get the picture.
Rigth now they personnaly hold those coins.
Once the ETF is approved, they will give them to the ETF, and in return the ETF will issue shares to them.
At that moment they will be the only ones holding shares.
Then they will sell them on the market.
Therefore, cash out.
Edit : it doesn't mean a single coin has to be sold nor move. 100% of the coins will still belong to the ETF. Only them will have a little less ETF shares and a bit more dollars.
This assumes of course that the Winklevii choose to cash out, if they do not then the ETF will have to purchase BTC on the open market to back up the shares.