theoretically you can reverse engineer more than a tech level ahead.
Take the whole of quantum mechanics or a modern microchip as a more precise exampel.
It is an obersvation of something happening. The why is still shrouded in mystery.
Basically someone observed that electrons behave in a certain way, others made use of the observation.
We use quantum effects on a daily basis, but why they work the way they do, we don't know. It was a long way of trial and error to get these things to work, but it clearly shows you don't really have to understand what is going on to use an effect.
So, if you present someone with a device, which is a ready made solution, reproduction of that solution is possible as long as you can observe what is happening.
The big point is knowing that it works, having a direction to work to makes all the difference in the world.
As a real world exampel:
NASA took the concept of the aculbierre drive to the lab last month. As crazy as it might sound, it is the concept of a warp drive, using a loophole in relativity to achieve speeds greater than that of light without violating einsteins laws. The theory is plausible and works on paper, but how a device that uses that effect might look like, how exactly you have to do it and if it actually will work in the end nobody knows.
Now take the hyphotetical scenario of presenting NASA with a finished drive using that concept. It would bypass decades of research just by showing that it works and how a device like that would look like.
Electricity is another exampel.
The basics of research on electricity date back to as far as the 17th century. Now go and present someone there with a ready made electrical device, build around the start of the 19th century.
Reverse engineering would be possible, the materials were there, what was lacking was knowledge on how it works.
Even though intially a 17th century person couldn't come up with advanced uses for it, presenting a working concept of a device using electricity (And some sort of power source) would jumpstart technology for at least a hundred years, if not a lot more.
Another scenario:
Civilization goes down the *******.... radio and electrical devices would still be there anyway. It's not so hard to build them after all, once you know it can be done and have a glimpse on how it works, basic electricity can be produced with pretty low tech all around.
So, to put it together: If you know it works and have a direction to look at, it all get's a big lot easier, even several tech levels ahead of you