A granite boulder has broken loose at the top of a mountain pass, and dusk is settling over the valley below. Everything that happens from here is down to two things: where you steer, and when you slow down.

Drag sideways to guide the boulder along the descending road. Hold the strip at the bottom of the screen to brake. That is the whole of the control scheme, and it is more than enough to keep you occupied, because the two demands pull against each other. Timber barricades block the pass, and only a boulder travelling fast enough will shatter them and carry on through. Speed is the thing you need. Speed is also the thing that throws you off the mountain.

The hairpins are where the game is decided. Come into one carrying the momentum you built on the straight, and the boulder simply keeps going — over the edge, out past the verge, down into the grey valley. Brake too early and too hard, and you arrive at the next barricade with nothing behind you, bouncing off the timber instead of breaking it. The run only works if you read the road ahead: build on the open stretches, shed what you must before the turn, and pick the speed back up on the way out of it.

Fifteen levels take the boulder down fifteen different descents. Each one arranges its barricades and its bends to ask a slightly different question of you. Some give you a long clear run to gather pace before a single heavy obstacle. Others string the hairpins close together, so the brake strip barely leaves your thumb and every metre of acceleration has to be earned in short bursts. Learning a level means learning its particular rhythm of hold and release.

There is a real satisfaction in the moment when it comes together — when you let the boulder run just long enough, feel the timber give way, and have the brake down again before the road turns. Get it wrong and you watch the boulder sail out into open air, which is its own kind of entertainment. Then you start the descent again, and this time you brake a fraction sooner.

Simple to pick up, awkward to master, and built around one honest tension: go faster, or stay on the mountain.