Before performing the handstand, pull down your pants just below the ass cheeks while attempting to conceal the action with your shirt. Then stand in front of a crowd, or a "target" and get in a handstand with your ass facing them, causing your shirt to fall and reveal your ass. Bonus points if you can pull off and maintain the goat during.
A handi-tard is the combination of handicapped and retarded. My friend accidentlly made up this term when trying to say both words at the same time, and that came out.
When I was at the park, I saw these people in wheel chairs and helmets and instantly I knew that they were handitards.