All of you guys quoting 2nd hand sources should just take a look at the link that I posted. There you can see for YOURSELF instead of believing somebody that is just passing the MYTH that they learned and never questioned.
Both sides flap. How are they synchronized? easy. The resonance of the horn (some say slots) actually takes hold and forces them to vibrate with the standing wave in the instrument. It is a simple system. To change the tuning resonant system, force is required. That is called "chops".
If you guys know it all, there is no need to ask the question. My link offers simple videos - no language barriers to overcome.