Comparators can overwrite strong redstone signals with weaker ones.

If you rebuild it like show in the pictures, it will "count down" very fast and overwrite the stronger redstone signal with the weaker one, until it is at 0, then the redstone with the current of 15 will overwrite the 0 with 14 again and so on.

(If some complain about this as a cool feature, you can achieve this effect without using bugs )