....can you tell me how the x-wrist bit in brackets is a self defence movement?....
Toby,

I think that the movements in SNT are not actually a literal translation of fighting techniques, but more ideas on paths along which you can take to generate force. This relates to and is expanded in chi-sau, i.e. (the common saying) when your rolling you should just shoot forward and strike when there is nothing blocking our way. However, we rarely have the luxury of picking an ideal line to take.
This is because most people train to defend against the most common lines of attack: e.g. the straight punch, hook, and uppercut.

I believe chi sau takes this one step further and shows us that there are infinite lines of attack. It can destabilize the preconceived idea of what certain techniques are and how they should be delivered.

Knowing how to exploit someone’s weakness, and generate enough force to cause an impact, however, is another mater altogether.

Chi sau can teach us to take the unexpected route, and remain safe.

SNT points out some kea areas and movements that we can utilize. Through experience of chi-sau correctly - being relaxed and structurally supported enough to feel when there actually are gaps, and having the ability to make use of those opportunities are useful tools.