Afterburner is an item you can get in the game Running Fred. It is also counted as a one-time use only skill.
The Afterburner is one of the most useful way for Fred to get a speed boost. This acts like a portable one-use accelerating tile, which let Fred to have an extra speed boost approximate to two speed up boost from accelerating tiles. This can let Fred run at full speed for a period of time, and is able to leave Grimmy far behind, saving you a life.
This can be useful if you want to earn achievements such as Close Calls, which need you to survive a close encounter with Grimmy, and is also practical when you are in midair. When you are in mid-jump, a use of an Afterburner will let you jump a larger distance, letting you fly across obstacles such as a pit filled with spikes.
You can also use Afterburners repeatedly, letting Fred run for a long time, and can be fun and challenging when you do so. However, beware that you run so fast, once you hit a pole, you will end up exploding into a mist of blood and gore. This also proves to be wasteful.
The Afterburner looks like a yellow-colored rocket propeller. It was unknown how Fred managed to use it, but it proved to be usable in Running Fred. You can get these special items through Mystery Boxes in Adventure Mode before the update, but now you must earn them through farming Skullies in Adventure Mode and Survival Mode and buy them from the Shop.