Negative splits are called when a person is running the second half of a race faster than the first. So, for example, if you run the first half of the marathon in 2:00:00, but then the second in 1:56:30, you run a negative split. Achieving a negative split is not easy, and it takes a lot of discipline and practice. Most people can't do it in their first marathon. But, generally, if you can hold back and keep the energy in the first half of the race to run faster in the second half, you'll perform much better overall.