Yep, that's because they are only using JUST BARELY enough to execute the techniques they want to use. Please understand that this is a BIG difference between using the least amount of tension possible... There's a big misconception here that a lot of guitar players have, and that is to use the least amount of tension ALWAYS.

That is only true when it's necessary (such as playing really fast licks, or string skipping with short runs on each string)

But for techniques like vibrato and mad bends, you will need to use a LOT of tension, just relax right after you play the note....

I guess I'm repeating myself, but I think it's worth it, since it's such a huge part of guitar playing in general, and it's how i managed to reach 600 npm (woot!) in like a year of starting left handed :P

i'm just saying that to give you some encouragement that it's THAT easy, if you practice the way I explained above! :)