When a male has mounted his partner and is thrusting his penis into the partners mouth, he removes said penis and moves it wildly from one side of the face to the other. The receiving partner then wildly follows the penis with his or her head trying to catch it. Imagine a dog trying to catch a bird.
As an extra note, the slapping of the face, cheek, and chin with the penis is acceptable/encouraged.
Playa 1: Did you hook up with that girl last night?
Playa 2: You bet your ass I did. I bird-dogged her for half an hour.