The jetpack is a modification to the Pistol that allows tees to fly. It is enabled/disabled through jetpack on/off tiles in maps. While it is enabled, the pistol becomes an automatic weapon.
Using the jetpack
Tees lose height quickly when they are not firing underneath themselves, which is a common issue new players have when learning to use the jetpack. You should fire most of the time, only stopping for very brief periods of time. It should also be noted that jumping cancels any vertical speed you had before, which is useful for recovering from a rapid fall.
Dragging a frozen tee with jetpack
It is possible to drag a tee while flying, which is used in some maps. The flying tee loses speed if they bump into the other tee so it is better to avoid bumping when you are starting to fly upwards. You can fall slower by bumping into the other tee, as mentioned on the hammerfly page.