Hi Martin and Wayne,
Thank you for answering my question. Here is the screen recording:
https://youtu.be/hOqK6oAqp18
As you see, the paint is like 100% weight. And the subtract brush is like an add brush.
This file is not from Wayne. I made it by myself. Maybe I messed up something on the way.
But it looks so weird. See if you can figure out what the problem might be. If not, it’s ok. There are no problem when I tried doing weight paint in some other files.