To do a Tail Whip (Flatland) you...
1. Swing your left or right foot over the frame and when reached a slow speed, jam it into the fork.
2. With whatever foot still on the pedal swing the bike in a either counter-clockwise or clockwise motion. (If right foot on swing counter-clockwise. If left foot on swing clockwise)
3. Once the frame makes a full rotation stop it with the foot you put in between the fork.
4. Regain your balance and ride away in glory.
The Tail whip is one of the harder advanced tricks and will take some practice to getting used to, but is totally worth the time taken to practice.
To learn a regular Tail Whip watch the amazing nicholi in the video below