How does a Thermal Release Device (TRD) work?
Either expansion/pop or fusible link melt are the two methods used. Fire sprinkler heads also use these devices.

The melting method is usually two pieces of metal joined by a calibrated alloy similar to solder that has a specific temperature where the pieces of metal debond allowing the safety mechanism to trip.

Expansion usually is a capsule filled with a liquid that's calibrated to burst at a set temperature allowing the safety mechanism to trip.

In fire sprinklers, the safety mechanism is allowing the seal to blow and water to flow. In furnaces, fuel flow cut, in this water heater, a damper to cut off air supply.

Here's a link to a safety bulletin (Rheem/Ruud)

Visually, it's pretty self-explanatory in the sequence below. Bulb pops, spring jumps out of the way and pin on spring loaded damper no longer is obstructed from allowing damper plate to seal combustion chamber.