Shifting a manual transmission involved a complicated sequence of pressing the clutch, matching engine revolutions and the shifting gear lever.
The HydraMatic transmission combined hydraulic operation of a planetary gearbox (allowing much shifting to be automated) with a fluid coupling instead of a friction clutch, eliminating the need for de-clutching.
That ’42 Olds is a beauty!
For the last 60 years I thought Buick pioneered it with DynaFlow (?).
Here’s a website that has a lot more about the history of automatic transmissions than you’ll ever want to know: https://ateupwithmotor.com/terms-technology-definitions/hydramatic-history-part-1/2/