Hmmm... this might be a cs only thing.
It increases (or decreases) the velocity with which you move through the air (so if it's set to e.g. 50, when you jump and hold forward while in mid-air, you'll move faster than you would normally when you jump). But it only affects the speed with which you move when you're in the middle of the jump. It won't affect the initial speed of the jump. This is I believe used in surf maps.
When you set it to negative, it gets quite funny. Because then if you run forward and jump while still holding forward, you will jump forward but then almost instantly change direction and start going backwards even though you're holding the forward key.
This setting is especially fun when used in combination with a low sv_gravity.