While all our our bunnies are certainly dainty little creatures with impeccable social habits and would never snarf their food like heathens (I can't hardly type that without bursting into laughter), rabbits can indeed choke. Items being consumed can block the trachea, causing a potentially life threatening situation.
This is rare, however it does occur, and knowing how to help may mean the difference between life and death.
Did you know that bunnies don't have a gag reflex, or the ability to vomit? It's true! Bunnies can cough, so a partially obstructed airway is probably something the bun can deal with themselves through coughing, shaking of the head, etc. But if the bun becomes distressed or cannot breathe, the airway may be completely blocked.
A rabbit with a blocked airway may exhibit a variety of behaviors; first they may shake their heads or walk backwards trying to "back up"; they may stretch up with their nose high in the air, mouth agape, trying to get oxygen. Their eyes may bulge. While this may distress you, it's important to remain calm, know what to do and to act fast.
By employing the same concept as the Heimlich Maneuver in humans, you can assist the rabbit by forcing air out the trachea and hopefully dislodging the offending item blocking the airway.
There are two ways to do this:
Hold the rabbit firmly, providing ample support for head and neck. With their nose pointing downward, firmly and gently apply pressure upwards against the diaphragm. Make a smooth movement that starts mid abdomen and sweeps upward toward their ribcage. This will apply pressure against the lungs, and by using the force of that air trying to get out, dislodge the item in the trachea. Having the buns's head pointed down will help it not fall right back into place again.
In a dire emergency, more force may be necessary. This method is more difficult, and you must be very careful to firmly remain hold of the rabbit (for obvious reasons). It also may not be possible on a larger, heavier animal. Place the bun, face down, between your forearms. Use your forearms to immobilize the bun's head, neck and spine so they do not move.
Raise the bun in the air and, nose pointed down, swing your arms gently down (remain in control AND in firm grasp of the bun, obviously). Doing this pushes the bun's internal organs up, against the lungs, and will push air out of the lungs to dislodge the item in the trachea.
You may have to do this more than one time, but stop immediately once the bun begins to breathe.
After any serious choking incident, get to the veterinarian right away. Additional items may have been fully aspirated (drawn into the lungs) and this, along with damage to the trachea caused by the lodged item, can result in pneumonia or infection.