While they are sometimes a bit more complicated, one of the advantages of smaller offers like a bet 5 get 10 is that they usually don’t have too many strings attached to them.
While it varies from site to site they generally follow the same formula, when you make a qualifying bet your account will be credited once the bet is settled: put simply, bet £5 on something, and when it’s done a £10 bet credit will be added to your account.