We express a lot through our hands.  For example, think about what your hands are doing at the dentist's office.  Are they relaxed, or are you gripping the armrest because of tension or stress (I personally grip the shit out of the armrests, even when I try to relax).

A woman's hands during sex are equally expressive and if they're just lying there limp, she's not really that into it.  If she's gripping the sheets or you, or showing some other reaction, then she's feeling it.  It's just one more "tell" to judge the response of your partner- obviously, there are others.  

Watch the hands of a porn actress if you want to see this for yourself.  Ignore the ridiculous soundtrack and just watch her hands.  Or listen to the sound track and see how totally out of synch it is with what she's actually feeling.  Now, some porn stars can actually get into it if the scene is allowed to run and the director doesn't stop the action 10 times for lighting or camera placement, and you can see that as well.

I'm sure the good porn girls know how to fake that too, but the average civie girl will tell you a lot with her hands during sex.

That I was first made aware of this about 15 years ago by a porn director.  On several occasions, I rented my airplane hangar, and the toys it contained, as props to a company that arranged locations for a group of erotic photographers.  

Naturally, I needed to attend the photo shoots to make sure nothing got damaged.  During one of the shoots, I had some time to speak with the guy that brought all the models, most of whom were moonlighting porn stars.  He had directed porn in the San Fernando Valley  (I live in So.Cal., so it's not that unusual).  As we killed time talking about some of the models, he told me that looking at a girl's hands during sex is how they can tell she's really into what she's doing during a scene, or if she's just going through the motions.

Despite the cheesy production values of most porn, they do realize that if a girl isn't into it, it will come across to the audience.  So they look at her hands to see what she's really feeling, because most actresses know to control their faces (it's acting after all).