The mouthwash solution is designed to lower the PH of the sodium chlorite almost to the point where chlorine dioxide is released. Actually, trace amounts are released, but we ignore those. When the solution encounters acid in the mouth, the activation continues and small amounts of chlorine dioxide are released. Those small amounts kill the bacteria and attack the biofilm that forms in the mouth.
If you have a lot of acid bacteria in your mouth, a lot of chlorine dioxide is released. If you have no acid bacteria in your mouth, virtually no chlorine dioxide is released. Chlorine dioxide can damage the enamel on your teeth, so this allows you to rinse often without worry about damage.
When you swallow a sodium chlorite solution your stomach acid activates it.