Some of you may know about it and some not, but this is a very fun trick to use in jump maps such as skyscraper/slide. This trick is completed with a medic and an engineer using the needle gun. Here are the steps you need to follow in order:

1. Engineer builds dispenser
2. Engineer then jumps on top of dispenser
3. Medic(s) crowd around dispenser
4. Engineer jumps repeatedly
5. Medic(s) shoot syringes at engineers feet

NOTE: 3 and 4 can go in either order.

This will get the engineer to places he was not meant to be! It also gives him the ability to build teleporter’s and other structure’s!

QUICK TIP:
If you want the engineer to go straight up, shoot at his feet from directly below him. If you want the engineer to go forward/backward, shoot at him in the opposite direction you want him to go from a distance. This can be tricky, but if you master it, you can shoot engineers all over the map!

UPDATE:
This works on any class, it just requires the user to be high enough over the medic to complete the boost. You can try this with a soldier after rocket jumping, or a demoman after sticky jumping.