Towards the end of Downhill Jam, you'll find the "ski jump" gap. If you've completed the career, it's the ramp at the bottom of the large rock where one of the valves you opened was located. When you go down the rock ramp, you'll go into a small underground section, before coming out and ramping down toward the end of the level. I wasn't able to get this gap to trigger until I jumped a little early, instead of riding the ramp all the way to the edge. With a character with max stats, this took one attempt to hit the 100 ski jump mark once I got it to trigger.