It was a jam chuck. They appear to be used extensively. Here's another video where they are used. I had the privilege of seeing the late Yasuhiro Satake at the AWGB seminar a few years ago. It was a real eye opener. https://www.youtube.com/watch?v=Cc4mD8t-xOs
In this one, you can see the jam chuck being used but also blocks of wood with some metal pins / nails which are driven in to the workpiece to hold it.