I posted this on Instagram, so let me post it here as well.

Here's the explanation:

This chronograph is used to time the start of a regatta, and operates in an unusual manner. When “resting”, all five discs are gray (to match the dial) and the second hand moves normally. When you press the pusher (at 4 o’clock), the second hand immediately returns to zero and the five circles turn red, and both the second hand and the discs begin the five-minute countdown, with one circle turning gray every 60 seconds. So it is a five-minute timer, with the flyback feature.

Jeff