Lets say a horse / team are drifting.
You lay £2 @ 5.0 to give you a risk of £8
market drifts further
You back £2 @ 5.2 to give you a return of £10.40
This means that:
if the horse / team wins you'll have a 40p profit
if the horse / team loses you'll have a 0p profit.
However, you can spread the profit by clicking the 'H' button.