The Jetpack upgrade is unlocked once the player completes Ghost Hunter Riley's 15th and final quest
The Jetpack greatly increases vertical mobility and activates once the jump button/space bar has been held and the initial jump has (or at least should have) ended. (i.e. to use it, just hold your jump button/space bar)
The Jetpack literally grants the user another jump after the first one has reached its peak (i.e. once your initial jump reaches its highest point, the Jetpack boosts you with another jump)
The Jetpack's color and amount of steams is dependent on the pack or battery the player has equipped.
The Jetpack can be used for some amazing glitches, most of them cheating on obbies or getting to places not normally possible.
And, with the exception of using it with a Hoverboard, the Jetpack can't activate midair and go it's normal velocity vertically.