The fish eye occurs when doing a girl from behind, or doggie style if you will, and then suddenly and without warning, attempting, purposely or accidentally, to lodge it in her pooper. She will most certainly become wide-eyed and turn her head to look at you in horror. From your angle ou would only be able to see one wide open eye - resembling a fish eye.
I was banging Gertrude from behind, doing some California long stroking, when I pulled it out too far and mis-judged my re-entry. Much to her shagrin, my meat missle lodged itself in her poop chute, and she gave me the 'ol Fish eye!