The hairspring is both the heart and brain of the watch. Like the brain, it is so complex the only thing we know about is what it does.
Bovet is one the very very few watchmakers that actually manufacture hairsprings. They are fitted onto the balance wheel which goes back and forth at a regular speed (its frequency). Those displacements are counted by wheels and transformed into seconds, minutes, hours and everything beyond.
The hairspring opens and shuts, yet always comes back to its original shape. It is impervious to temperature changes, which is awesome at 0.03 mm thick. It is barely affected by air pressure nor magnetic fields. And it is rust free. That 1 mg gossamer thing remains almost-everything-proof, while beating 500 million times a year, for decades on end! It does require all of Bovet's experience to achieve such a result.
