“虛領頂勁“ is one of many Tai Chi principles. 虛 means void, which really has misled many people.
Because 虛 is the first word, people believe the neck should be loose and void. I believe the truth is in the word 頂, which can be a noun “on the top”, or a verb “stand up” and “gore.” Based on Wikipedia, “gore” refers to the act of impalement on an animal horn, especially as in bull fighting. I like that. Master Chen talked about this not long ago, using the rubber band to visualize the 頂. We have been working on this ever since. The benefits are obvious. With 頂, you establish a strong center line, create a yin yang split on your upper body (combined with “Qi sink to Dantian” 氣沈丹田), connect your hand with your feet. The end result is that you can direct the opponent’s force to the ground as soon as there’s a contact. Personally, I think this is one of the most fundamental improvements for me.