When you stick your arms out in front of you as if you were dirtbiking, then pretend to ride a dirtbike by running around and making loud and obnoxious dirtbike sounds at the top of your lungs in a public area. May be accompanied by dirtbike gear. Usually, a bout of dirt biking is accompanied by wipe-outs, where you do a spectacular crash and make sputtering sounds. Afterwards, you can pick up the bike and do a sweet wheelie to get out of there.
Guy 1: Man, did you see that plank I did?
Guy 2: Planking? Everyone knows dirt biking is where it's at!