For this market Spread Betting Firms predict the number of runs that will be scored in each session of a typical Test Match.
Take an example of the opening day of a Test between England and Sri Lanka.
SportingIndex might predict that 78 - 82 runs will be scored in the second session of play (Lunch until Tea). With the lunchtime score at 70 for 1 and the weather set fair, you may believe that batting would become easier in the second session, so might bet high at 82, for the stake of your choice, in this case, £5 a run.
If 102 runs were scored in the second session you would have been proved right. Had you gone high at 82 you would have won 20 times your stake: (102 - 82) x your stake, 20 x 5 = £100. However, had a couple of wickets fallen and only 70 runs been scored in the 2nd session, you would have lost 12 times your stake: (70 - 82 x your stake = -12 x £5 = -£60.