When you run down a slope in Quake, gravity is on your side. You can run a little bit faster horizontally as a result of the downwards force, just as in reality. The greater the incline, the more noticeable the effect. If it's too steep, though, you'll actually leave the ground. Similarly, you go a bit slower when running up a slope, slower still as the gradient increases, until eventually the hill is too steep to climb at all. All sensible enough.
However, there is a way in which you can take advantage of the physics here, since Quake doesn't take slopes into account properly when you make a jump. Whenever you jump up, Quake just increases your upward velocity by a fixed amount. (270 pixels per second, in fact.) If you already have some upward velocity as a result of running up a slope, the jump velocity is simply added on. This means that by jumping before you reach the top of a slope, you can reach a greater height than if you jumped having reached the top of the slope.
The original Qtest was stranger still. In that version of the game,
whilst one was in the air, the vertical movement keys worked just as
they do when you are in water or have activated the FLY
cheat. This meant that you could actually press moveup to
go higher into the air when you jumped. Gravity was still greater than
your upward speed, so you didn't disappear up into the atmosphere
forever, but you still jumped appreciably higher this way. So the
remaining anomaly is a very small bug in the physics of Quake compared
to the Qtest behaviour.
But it is a useful bug nonetheless. You can use this effect to make jumps to otherwise unreachable heights; for example, you can use the little slopes on the architectural conceits underneath the big red window in the start room of E4M3 (The Elder God Shrine) to get straight up onto the ledges that overlook the room by jumping at an awkward angle.
You can also combine this extra upward velocity and jump with an explosion to make a higher rocket or grenade-jump than you would otherwise be able to. The canonical example of this is the difficult grenade-jump from the gold key to the gold door in E1M3 (The Necropolis.) The fact that you had to use the little slope on the platform to achieve the height you needed made this one a real challenge in Skillz Test #4.