On any level in Halo 1 you can jump off a large cliff and land on a very slanted surface, one that you wouldn't be able to stand on if you were down there. The reason for this is that there is no friction on those surfaces. If you do it right you won't get hurt. Usually it works best if it is a very slanted rock, or a wraith. If it is too far, though, you will die. A good example of this is when you are on a bridge.
An example of the wraith is: on the level Halo when Cortana says to go to the third level you can jump down and land on the front, slanted part and you won't take damage. This can be used to shortcut some levels. During the level Two betrayals when you are going down to get the banshee, you can slide down the slanted part of the walls and effectively skip the walk down Another example is on the level Keyes when you are on the third floor and have to get to the banshee, jump down onto the second floor, kill the elite in the shade, and try to jump onto the Banshee to avoid fall damage, avoiding all the flood/covenant the long way offers. If you get into the banshee immediately, you will skip to a cutscene and avoid fighting the two elites there.