If you hold a button (F, I think), then your heat bar will start to fill up and once full, you will enter Full Steam Ahead!
It is a really poorly-implemented feature, though. It gives a 50% speed boost, with the drawbacks that it kills your handling, drastically reduces fuel efficiency, maxes out your heat, eats a chunk of fuel up-front, disables your weapons, prohibits interacting with objects in the world, massively increases collision damage, takes a really long time to activate AND deactivate, and plays an annoying overdramatic animation on startup. The speed boost itself doesn’t feel all that impactful and due to the handling is useless in most situations.