Throttle is the control that affects the speed of an asset. Throttle up is [w] and reduce throttle is [s].
A mech's and aerospace's default throttle is gradual and incremental. By pressing throttle up, the throttle will slowly increase until the button is let go, at which point it will stay at whatever point it was when the player most recently adjusted it. If the button is held down, then the throttle will increase until it reaches 100%. Reducing the throttle behaves in much the same way. Once the throttle has been reduced to 0%, any further reduction will result in reverse throttle for mechs, and reverse thrust for aerospace, so long as they are on the ground. To make the throttle instantly transition from 0% to 100%, press [k] to enable "throttle decay." While throttle decay is enabled, letting go of [w] or [s] will result in the asset's speed gradually falling back to 0% throttle.
A mech's throttle can be temporarily reduced (even as low as 0 kp/h) when it stumbles into a wall, walks steeply uphill or downhill, walks over very uneven terrain, falls too far, or collides with an ally or an enemy. The throttle's top speed can also be temporarily reduced when a mech overheats or permanently reduced (until repairs can be found) if its legs or back torso are heavily damaged.
A tank's throttle can be temporarily increased when going downhill (well over 200% for steep enough grades) or temporarily reduced when going uphill or collides with an ally or an enemy. The throttle's top speed can also be permanently reduced (until repairs can be found) if a tank's back armor is heavily damaged.