This is fascinating.

The Gilbreths identified and labeled 18 movements and decisions that a worker does into “therbligs,” a word that’s just their last name backwards (with the “th” taken as a single letter).

With a task broken into therbligs, you could look at delays, when and why they happen, and try to eliminate them. Frank Sr. gives the example of shaving in Cheaper by the Dozen:

"Suppose a man goes into a bathroom and shave. We'll assume that his face is all lathered and that he is ready to pick up his razor. He knows where the razor is, but first he must locate it with his eye. That is "search", the first Therblig. His eye finds it and comes to rest -- that's "find", the second Therblig. Third comes "select", the process of sliding the razor prior to the fourth Therblig, "grasp." Fifth is "transport loaded," bringing the razor up to his face, and sixth is "position," getting the razor set on his face. There are eleven other Therbligs -- the last one is 'think'!"

The 18 therbligs, via Wikimedia Commons.

To better document and present a process, in 1924 the Gilbreths came up with the “flow process chart,” where specific shapes indicate specific actions. As they described it, the process chart “presents [data], in simple, easily understood, compact form,” and it was a useful tool for improving efficiency in “production, selling, accounting and finance.”