Originally Posted by steelbikeguy

Derailleurs have always been about the search for a good way to shove the chain from one cog to the next. Some have been more sophisticated, and some less so....
Definitely get a copy of the Dancing Chain! That will keep you entertained for a long time, and thoroughly explain all of the ways that have been explored on the path to our present technology.

As a teaser, you might want to look at the Vittoria Margherita. This was a derailleur from the 1940's that just used some metal fingers to shove the chain left or right as the rider backpedaled.
https://steel-vintage.com/taurea-vit...ad-bike-detail
The Campagnolo Cambio Corsa is another example where metal guides were used to move the chain left and right. The novel feature is that the proper chain tension was achieved by allowing the rear wheel to roll fore and aft in the horizontal dropouts while changing gears!
Dave Moulton's Blog - Dave Moulton's Bike Blog - The Cambio Corsa: Campagnolo's Early Masterpiece

As far as the usual 70's derailleurs, they do require more attention and care when shifting. The chain isn't as eager to hop to the next cog, so you typically need to move the lever beyond the final position in order to get the chain to make the change. Indexed shifting definitely made the process fairly mindless and something beginners could do well.

Steve in Peoria