In an inactive state, sound transmitted from port 1 is audible to ports 2 and 3 at equal volumes. Sound waves bounce back to port 1 as an echo as well.
When the system is running, however, only port 2 hears port 1's sounds.
The trick is to blow swirling air into the cavity at a specific speed and intensity, which allows the sound waves to synchronize in a repeating pattern. That not only guides the sound waves in a single direction, but gives more energy to those oscillations so they don't dissipate. It's kind of like a roundabout for sound.