I did not find another categoriy, so if this quick tutorial is wrong here, please give me a hint and I may repost it:
The player launcher does have the option to "AddToPlayerVelocity". In a case of a trampoline this is working the wrong way: Your movement while jumping on the area is falling. So it will be subtracted from the jump height instead of making your jump more powerfull and higher.
So you should change this:
currentVelocity = other:GetVelocity()
into
currentVelocity = other:GetVelocity() * Vector3.New(1, 1, -1)
Greetings,
Marcel