"Magic Skeleton" ETA 2824-2

ETA 2824-2 without calendar mechanism
Twice a minute one can admire the ETA 2824-2 without calendar mechanism. Then very slowly and gradually, the second hand disk will turn to almost completely black.
What I still could not figure out is how they managed to synchronize the phototropic changing to the mechanics of the watch. The face is always clear at sharp 00 and 30 and always black at 15 and 45. The 2824 is a hacking movement, I had expected to see that the disk would continue to change its opacity once the movement was hacked. But much to my surprise, the disk will stop at the exact level of opacity if the watch is hacked and only resume its changing once the crown is pushed back in.

That is the beauty of posting watch pictures on fora. Only minutes after I posted the pictures on the web, fellow PMWFer Dan aka Zeb had the following idea, and I think it makes a lot of sense:

"There are 2 polarized discs, one fixed and the other rotating with the second hand. Two polarized lenses will let light through when they are held in the same orientation, but when you rotate one 90 degrees they block the light completely. This would correspond perfectly to the 15 second change.

What do you think? It's the only non-electric mechanism I could think of.

Dan"

Thanks Dan! That could indeed be the riddle's solution! - Confirmed by Wee Meng on August 16th, 2004 per email. Thank you Wee Meng! Most kind of you!