Skiing is a feature that was invented during the Starsiege: Tribes beta. In Starsiege: Tribes, also known as Tribes 1, skiing involved jumping immediately as you touched the ground to prevent friction from slowing you down. There was no air resistance, so by jumping in this manner, you could retain 100% of your speed an momentum while traveling across a flat surface. Additionally, if you are jumping forward, you will continue to pick up speed with each jump, to an almost unlimited velocity. If you are traveling down a hill, you will keep all of the speed that you gain from gravity pulling you down that hill. Eventually, a script was made in Tribes 1 that automates the act of skiing.
In Tribes 2, skiing was built directly into the game. If you hold the Jump key (spacebar by default), you will automatically begin to slide across the ground without losing (much) speed. The amount that you slow down or don't slow down is dependent on what mod you are playing. In Base, you will still be slowed by friction. In Classic, you won't, just like in Tribes 1.
Deadstops
One thing you will inevitably run into are deadstops. These are an unfortunate byproduct of the original Torque gaming engine on which Tribes 2 now runs. Basically, if you are skiing along a completely flat surface, you will sometimes come to complete stop as if you have run into an invisible wall. You will often even take damage when this happens. Deadstops usually only happen when you are moving fast, and therefore you will rarely come across one in Base. However, in Classic and Version2 it takes a lot of luck not to run into a deadstop at least once per map.
Deadstops are not random. They will always be in the same place every time on each map. If you really are bothered by them, you could memorize the location of every deadstop and simply avoid those areas or make sure you are not touching the ground if you fly over them. Sometimes this can be incredibly difficult, as it is on the map Damnation, but they are unfortunately a part of the game.