A lot of swinging and swaying on his back and shoulder movements. Obviously I'm not knocking Arnold, but why do you hear it is so bad to have momentum going in your lifts, yet you see the greatest doing it?
A lot of BB's preach good form, but i've seen videos of Jay, Viator, Victor Martinez etc all training with sloppy form. I think we all cheat a little at some point or use less than brilliant form. However momentum can be a good tool for shocking a muscle. I've found this with cheat curls and calf raises etc.
I know I wasn't there - or even born - in that era, but you do get a warm nostalgic feeling looking at footage that old with athletes that good. Maybe it just reminds me of when I started lifting, who I was inspired by, who I wanted to look like as well as remembering the good people I've trained with over the years.