When a male is approaching the female from the backside to have sexual intercourse, he tries to insert the penis into the anal region thus resulting in a look back from the female. Her head will be tilted from the left or the right thus straining the eyes in order to view you. This results in the look of "fisheyes".
Check this out...I had my girl bent over doggystyle and i tried to give her a poke in the recroom. She gave me the "fisheyes" and I knew she wasn't going to have anything to do with that.