Is it necessary to explain why a futuristic device works the way it does?

I misunderstood you at first, sorry. I was referring to the case when a writer creates new rules halfway in the book as the plot demands (often breaks previously established ones as well), or simply "forgets" to establish the rules. This is what I call lazy writing and I have seen this too many times
As I stated earlier "at least a short sentence should be there as proof that the writer haven't simply forgotten the possibility to jump there directly" <-- establish the rules.

NOW I agree with you. Over-explaining things to make a book more interesting makes it boring. It's telling and the reader does not want that. As @jazzabel pointed out :

 

In Michael Crichton's Sphere, the futuristic device and powers don't have explanations as to how they work, and they seem magical. He said himself that we wouldn't understand future technologies. Think about going back 500 years and showing someone a mobile phone - they'd think you were some sort of god. If you rang for pizza you'd probably become their new master

 

It all depends on what you think; does it make sense to give a full-blown explanation in your story? Or can you let readers go crazy with their imaginative little minds?