Weight sled sprint. and backwards sled sprint

I have noticed this in the WOD's several times, and i find them interesting, but i want to make sure i do them right. Are they to be performed up an incline, such as a hill, or can it be performed on a flat ground?

Flat, but uphill would be interesting to say the least.

Thank you cole!

Up hill ones rock. I use it a lot. Bringing the weight back down the hill sucks though.