When Bookmakers offer free-bets, most of the time they will be stake non-returnable. This is because people can simply use this free bet on a certainty, such as Chelsea to beat Rushden and Diamonds. They would then receive the entire free-bet back, as well as any other winnings. However when the stake isn't returned on a bet like this, they would hardly earn any money from it. For example if the odds on Chelsea to beat Rushden and Diamonds were 1.2 and you had a £20 stake-not returned bet and you placed the £20 on Chelsea to win, you would only receive £4 back. Obviously if the stake was returned you would have earned £24.
The Bookmakers don't want people to be able to do this, which is why the free bets or bonus bets are usually not stake returned. However, the best way around this is to bet on something with high odds when it comes to using a free-bet.

For example, say you had gone the other way and used the free £20 bet on Rushden and Diamonds to beat Chelsea at odds of 9. You would have won £160, and this doesn't include our original stake. This is a very good return on the free-bet, and is a lot better than the £4 you would have won if you backed Chelsea. Obviously it is very unlikely that Rushden and Diamonds would have won the match in the first place, but as long as you remember to Lay the bet at the Exchange, you would have unlocked most of the free bet as your own money (as long as you found odds similar, or lower, when laying the same outcome at the Exchange).

From the matched bet above,you can see that the Lay Stake needed to match this bet is £10.29 and that there is an overall closeness of %97.79. The Lay Odds in the situation above are lower than the Back odds, by 0.05. This would normally form an Arb*, but due to the %5 토토보증금Commission, there is no arb. Often when you place a qualifying bet, and then lay it out, you will make a small loss as long as you have odds which are reasonbly close together. However on the odd occasion you will get Lay odds which are lower than the Back odds. This means that no matter what happens in the event*, you will come out with a profit. Often though you only ever make a small loss when matching the bet, but this is okay because by matching the bet in the first place you will be unlocking a free-bet or bonus. The free-bet or bonus is the earner, and you can usually gain about %80 of the free-bet as your own money (as long as you match the bet properly).