Dune

hold for gas. let go before the top.

race a rider in black over the dunes: hold for gas, ease off before each crest or fly nose-first into the next one

Over the top of a dune the sand has to keep bending your path downward, and it can only do that as hard as gravity pulls. Once your speed squared times the crest's curvature beats g, the ground drops away and you fly; a sharper crest is a lower limit. A landing keeps the speed along the sand and loses the speed into it, which is why riders ease off before the top.

Concepts: centripetal-force, gravity, projectile, momentum.

An interactive you play with one thumb, on Wundr.