It is possible to bunny hop your dirtbike. While not moving, hold L1 (clutch) + X (gas) + Analog-stick Down. Then, hold R2 or L2 (preload) until the bar is full. Then, release L1 (clutch) and if done correctly, you should take off into the air. Level yourself out and complete the bunnyhop. It may be a little tricky at first, but it is fun to jump on buses and and other things. You can also bunny hop, going slowly (or any time you can pull a wheelie). However, it is easiest when going very slow or staying still.