An incidental, innocent gaze at someone where your head has to turn less to see them, than their's does to see you. More head turn means less staring rights, for example. Adds more uncomfortability factor if you have to turn your head to gaze at someone, they notice you looking and they're WTF'ing at you.
Best done while waiting for a train or something that requires glancing in one direction continually where the person you want to check out is also in that direction.
I was at the diner yesterday and I tried to check this girl out a bit better, but she noticed me looking over and had the staring rights on me.