These pictures are really amazing.  First let me explain what is happening in the scene.  This is a side view of Briana Banks in the missionary position being fucked by Evan Stone.  In the first few seconds of this shot you see her screaming "OH FUCK!"and then she takes a few thrusts with her eyes squinted and mouth closed, she is being fucked good but with thrusts with moderate pullout length.

In the picture at left she lowers her head a bit as if she is looking at his penis going into her pussy and suddenly she starts opening her mouth.  He has pulled out of her quite high this time and she is probably gasping looking at the entire length of his cock above her about to thrust long and hard into her.  I think she looks terrified!  Having seen the size of his cock earlier in the scene and later I can't blame her.  She looks like someone about to be stabbed, the shower scene music from Psycho almost comes to mind!  She is bracing herself for discomfort and pain but at the same time she wants it bad and that is what makes it sexy.

In the picture at right you see her expression at deepest penetration of his penis thrusted into her pussy.  You can tell it was every bit as hard as she was anticipating it to be.  Her entire body was moved back about a couple inches, her boobs bounced up a bit and you can see how far his body has been lowered onto her.  She has opened her mouth wide and she appears to be recoiling in whorror (a word I made up just for this scene meaning a cross between fear/discomfort/pain and raw sexual desire/pleasure).  She is lets out a whale of "AAAHHH" at the same time you hear the pounding of flesh between the two of them.  She must really be feeling the friction of his long cock going through her wetter than hell pussy, the tip of his cock like a battering ram pounding her cervix.