The afterburner is capable of giving a ship a short boost in speed and acceleration, taking power from the engine. On most ships, the afterburner speed is at least double the speed obtained from the drive alone. The Drayman cannot have an afterburner and the afterburner cannot be used when the ship is in Autopilot mode.
The afterburner is activated by pressing and holding the Tab key while in flight. This drains energy for the front gun and eventually part of the shields if held long enough. Release Tab before engaging the Autopilot with A or you might arrive with weakend shields.
It is highly recommended for most players to purchase an afterburner. The starting Tarsus already has one. The extra speed is useful moving from Nav point to Nav point, and it can provide a serious edge in combat. However, use it sparingly during combat, as it is very easy to drain all your power boosting and not be able to fire once you reach your target. Once the player upgrades from the Tarsus to another ship that can equip a higher Engine Upgrade, the afterburner won't drain engine power as much.