How do I properly disc jump?

One of the core gameplay elements in the Tribes series is trading health for speed by disc jumping. I seem to be having trouble pulling this off though -- I've tried shooting straight down as I come off a hill, as well as turning around and shooting slightly behind me, but I always end up losing more than half of my health for a minor speed boost at best.

Am I doing something wrong, or is disc jumping simply not as effective as it was in earlier games in the series? (I'm a Tribes 2 veteran, so I'm still used to Classic physics.)

Coming from Tribes 2 myself I have to agree with you that the physics in Tribes:Ascend do differ greatly with respect to disc jumping. In respect to old school vs. new school, T2 had much more of a 'lofty' (more air control) element to its physics in comparison to T:A (almost no air control). The key in this new iteration however is timing and of course practice.

One thing to note before explaining is that the variances between the projectile speeds of the variety of disc launchers in this game, does require that you're familiar with each specific weapon. For example, the Infiltrator's disc launcher has a slower projectile speed than the Soldier's disc launcher. All disc launchers now have the same projectile speed. In addition to this other non-disc weapons (with unique timing) can create a similar effect such as the Pathfinder's bolt launcher. This timing will be the key to achieving the maximum boosting effect from your weapon's Area of Effect (AOE).

The best way to practice this is to try honing your skills in the free roaming portions of training mode. The key to getting an effective boost is to time your shot so that you are at the minimum distance from the top of the hill (or any surface) to get the full effect of your weapon's AOE. By this premise you should be able to jump/jet a reasonable distance away from any surface and receive an appreciable boost.

For example: When skiing up and/or approaching a hill at adequate speed, upon reaching an angle in which gravity begins to diminish your speed, using your jetpack to maintain constant or increasing momentum is of course the common practice. However, when doing this, your distance from the ground may not change greatly or at all due to the angle of the hill (and gravity). Firing your weapon at the ground from a close distance will not be effective. The best strategy is to continue jetting up the hill, maintaining speed until you have risen from the ground a reasonable distance. At this point firing at the ground (within the effective AOE range) will cause the player to inherit a significant boost. Judging the firing distance is more an art than a science and getting it right can make a big difference. With discs I have found that it is most effective to aim straight below the player when firing. Angular or rear-facing shots seem extremely ineffective. Practice makes perfect! Watch your speedometer when holding down the ski button to judge the effectiveness of your attempts.