The late lamented Ken Cottier couldn't sit in seize because of arthritis resulting from an old knee injury, and found it uncomfortable to move between sitting and standing, but when you practised with him you always had the feeling that this was someone who could kill you faster than you could blink (but, luckily for you, chose not to).
I took ukemi for Ken Cottier Sensei at a seminar about ten years ago. He gave me this "shocking" technique that dropped me straight down. I had no idea what he was doing at the time. Looking back, I think he was sharing some of his most potent knowledge with anyone who was interested.
Looking at the video, Jan obviously learned a lot of old style stuff, very daito ryu, so a lot of these old timers were exposed to a lot of deep stuff, and, contrary to some theories, maybe it all didn't go over their head. The late Alan Ruddock and Henry Kono also come to mind in this regard, Mary Heiny too. So the questions I have are ; where did Jan learn this stuff and who taught him? And why did he choose to demonstrate this stuff at the apex of his career?