Page 128, Chapter 19 Hand Techniques – Bong Sao should never remain – it is traditionally said. But its very common to see bong sao used as a block and the elbow left hanging in the air. This is an example of adhering to the principle of not allowing bong sao to remain.
The other thing to point out is the adherence to the “three do not” that Ip Man used to remind students in class but nowadays the “three do not” have seem to become the “three must do”. Interesting how Wing Chun has evolved as we move further and further away from the source.