The liability is the amount you stand to pay out in the betting exchange should you lose your lay bet.
However, if you have to pay out this liability, you will win an equal amount in the bookmaker from your bookmaker trigger bet winning.
This is the reason we use the Calculator, because it gives us the exact amout for our lay bet to ensure regardless of the outcome, the win/loss from our bookmaker trigger bet will equal the corresponding win/loss from the betting exchange.
So for example, if we placed a £10 trigger bet on Man Utd to win at 2.5 odds and lay odds of 2.5 also. We will place an equal £10 lay bet (or very close to £10 depending on the Calculator). This lay bet will have a £15 liability as shown in the Calculator and in the betting exchange when we enter our £10 lay bet stake.
If Man Utd win, we win £15 profit from the bookmaker bet, however we will lose £15 in the betting exchange (our liability).
If Man Utd do not win, we lose our £10 trigger bet from the bookmaker bet, however we win £10 in the betting exchange (our lay stake).
So regardless of the outcome of the match, we neither win nor lose, hence this is risk free. However, we still trigger the free bet!