Betfair's Cash Out is a useful and potentially profitable facility.

The idea is that if you place a bet on an event and before it concludes the odds obtainable have shortened, you can take a guaranteed profit by allowing the system to place a lay bet at odds lower than your back bet. Thus ensuring a profit.

The best bets for this process I find are bets on politics which have a long time period. I currently have some bets on the American election e.g. Donald Trump for Republican nominee and Trump and Clinton for next US president.

As Trump wins more primaries his odds for both events will fall resulting in the option of cashing out of the bets for a guaranteed profit.

Ucantlose have developed a useful tool which calculates the return on a cash out including any commission payable to the exchange.

Try the calculator here