How does the Principle of 'Deliberate Practice' apply to the coaching of Rugby Union?

So lets look at a definition of 'Deliberate Practice':

‍“It is activity designed specifically to improve performance, often with a teacher's help; itcan be repeated a lot; feedback and results is continuously available; it's highly demandingmentally, whether the activity is purely intellectual or heavily physical; and it isn't muchfun.” - Geoff Colvin

...and a meaning of the 80/20 rule from the book 'Practice Perfect':

‍“The 80/20 rule is sometimes known as the "law of the vital few": 80% of results turn out tocome from 20% of the sources...It suggests that to become great, you should focus moreon practicing the 20% of things that most create value than the other 80% of things youcould plausibly spend time on...With practice you'll get stronger results if you spend yourtime practicing the most important things.” - Doug Lemov, Erica Woolway & Katie Yezzi

So how do you find the 20% of the things that are the most important to practice...?

‍“...experience and data can go a long way to helping you identify this 20%....if clear data is not available, consider harnessing the wisdom of crowds.” - (p.31, PracticePerfect, 2011)

‍Ask a group of coaches to name their top 5 most important areas of the game and thentake the average...

‍So the main areas applicable to help identify the 20% for Rugby Union are:

1. The longer you have been coaching/teaching...the more matches you have watched, the more matches you have observed, the more Conferences/Seminars you have attended, the more reference points you'll have to be able to identify the most important things to practice...

‍2. Recording, Analysing & Interpreting the Try Scoring Trends of tries scored in matches...will help you identify the main reasons why tries are scored...

‍3. Analysing & Noting the demands of Rugby...will help you identify the fundamental techniques, skills and actions of the game...

‍4. Seeking the opinions of a good number of coaches from all levels of the game...will help you come to a conclusion to identify the most important things to practice...

In essence: The “core” ‘Principles of Play’ (Games Understanding i.e. one in Attack and one in Defence) and the fundamental "core" technique (i.e. skill)...of Rugby Union...