This market, as the name suggests, is the Spread Betting company's prediction on the aggregate total of the winning margins of each race at a meeting.
The maximum winning distance for a Flat race is set at 12 lengths while for a National Hunt race it is set at 30 lengths. This also applies should only one horse complete the race. Distances under a length are recorded as follows: Short head = 0.1 of a length, head = 0.2 of a length, neck = 0.3 of a length, half a length = 0.5 of a length, three-quarters of a length = 0.75 of a length.
Here's an example. At a jumps meeting at Aintree, Sporting Index might price total winning distances at 64-69. As it had been raining non-stop for the previous two days, you might decide to place your spread bet high at 69 for say £2 per length, reasoning that heavy ground can often lead to big winning margins.
In the end the winning distances made up at 78 lengths. Had you gone high at 69 you would have won 9 times your stake: (69 - 78) x your stake = 9 x £2 = £18. But if winning distances had made up at only 60 lengths, the same bet would have lost you 9 times your stake: (60 - 69) x your stake = -9 x £2 = £18.