Hill Start Assist is a variant of Hill-holder

Hill-holder

Hill-Holder is a name for the mechanism invented by Studebaker that holds the brake until the clutch is at the friction point, making it easier to start up hills from a stop in manual transmission automobiles. It was first introduced in 1936 as an option for the Studebaker President...

 used by some semi-automatic, clutchless transmissions

Semi-automatic transmission

A semi-automatic transmission is an automobile transmission that does not change gears automatically, but rather facilitates manual gear changes by dispensing with the need to press...

. The system prevents the car from rolling away when trying to pull away on an up or down gradient, simulating a "handbrake hill start" manual drivers will be familiar with. The system engages automatically when a gradient of 3% or more is detected; it then acts to hold the car stationary for two seconds after the brake

Brake

A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....

is released giving the driver time to apply the throttle.