I'm surprised how much attention this got. Here's a mind-blowing (to me, at least) animation of the mechanism. And an article about how it works:
https://hobbycouture.com/en/a/how-does-a-sewing-machine-workThe old sewing machine is a Singer Slant-o-Matic from the 60's. It's electric, but the electricity only does one thing in the mechanism: turn the main shaft. Everything else is mechanical, and the main shaft can be turned by hand as well.
I darn'd my pants to heck. Then they tore again just outside the darning. Material's just too thin and worn. Makes me want denim pants. I'm also thinking of trying patching them so as to have them looser at the knees, to accommodate my autist way of sitting.