Needle boosting

From TF2 Wiki

Needle boosting is a situationally useful game exploit by which a friendly Medic or group of Medics uses the knock back from their Syringe guns to launch a teammate high into the air. As of a patch released on December 31, 2007, the needle boosting exploit has been repaired on the PC and is no longer possible.

This technique is used almost exclusively on Engineers because this class makes the technique both easier and most useful. When the needles hit an airborne teammate they produce a knock back effect that pushes the teammate in the direction that the needle was traveling. Therefore it is best for the Medic to be under the player that they intend to launch skyward. Engineers make this part of the technique easier because they can build a Dispenser, stand on top of it, and then start jumping while the Medics crouch underneath him. (See the video to the right.) When the Medics start firing upward, at the target's feet when he's airborne, they should be able to boost him significant distances. The boosted player has some control over the direction as well.

This is a useful strategy for rocket jump maps such as Skyscraper as it allows the Engineer to create Teleporters for short-cutting and Dispensers to replenish Health lost during jumps. In other maps however, it can be used to allow the Engineer access to places where he should not regularly be able to reach or build upon and in this situation the strategy is no longer considered legitimate.

Of historical interest, this is similar to the "sniper launching" exploit found in Team Fortress Classic, where a player would stand on a friendly sniper and who could send them great distances with a fully-charged rifle shot.