:

If we assume that the central track is seconds that makes the hand sweep round 4 times in one minute. Now if we time 10 events (bottles filled, items stamped out, 100m marker posts) then the outer track indicates how many of these we will get in one minute. If we time 30 events the two middle tracks (connected at 80, "6 o'clock") will indicate how many we will get in one minute. The outer of the two on the first pass of the hand and the inner on the second pass.

On the right in this catalogue illustration is something similar for checking rowing stroke rate.

and a page of similar industrial timers from http://www.onthedash.com/Guide/_Catalogs/1968-69_Catalog_(German)/

Mike