This is fascinating, but I'm not nearly sewing-literate enough to fully follow it. This is the best resource I can easily find for exactly what is being discuss

This is fascinating, but I'm not nearly sewing-literate enough to fully follow it. This is the best resource I can easily find for exactly what is being discussed: https://seekatesew.com/sewing-101-ease-sleeve/ https://seekatesew.com/sewing-101-ease-sleeve/ I guess people literally make the circumference of the arm hole in the sleeve smaller than that in the torso portion, and compress it to fit in the process of sewing them together.