This video shows it much better than I can explain it:
... BUT I'll still try.
Basically, if you land on the trampoline while the jetpack's trail is active (fly high up and then fall down while holding space), it will continue to show the trail without having the boosting effect if you continue to hold space.
only the first part of the video is me showing the bug, the rest is me messing around with the trail, because it looks kinda cool...
also ignore the fact that it has the GeForce Experience overlay, I didn't feel like editing that out