A vehicle's mechanical 'wig-wag' low air pressure warning drops its arm into view. What triggers this, and how does it reset?

AAMVA CDL Manual, Section 5, subsection 5.1.11 (p. 5-3), 'Low Air Pressure Warning'

Some vehicles use a dashboard light and buzzer, others use a wig-wag, and some have both; knowing how each type resets tells you whether seeing the warning go away actually means pressure has recovered, or whether it was simply reset by hand. A manual-reset wig-wag that's been pushed back up can give a false sense of security if a driver assumes it only resets when pressure is restored.