Hi Chris,
You wrote:
Say for instance that if your objective is to kick a door off it's hinges, your training to improve your speed/explosivity using plyometrics or other means will help with initial part of the movement and with the movement itself, but at the moment when your foot makes contact with the door, that's when a strong isometric contraction can make a difference. It's what I've been thinking, although I don't know if this line of thinking is correct.
Chris, I think your line of thinking is right on the money. I synergistically apply plyometric and isometric exercises as punch-specific as I can for the above rationale among other things. You and I are on the same wavelength my friend. Great minds think alike...

In regards to the book you mentioned, I have it (and all his other books for that matter). They're okay if you already have a background in boxing. In my opinion, Thomas' books are not geared for a beginner at all.

While Jay Thomas briefly delineates this methodology in his books, I actually learned most of what I know about punch isometrics from my boxing coach Solomon Johnson and a friend of his, the late, great Ol' Mongoose Archie Moore. Back in 1981 or '82, Archie was in my hometown for a week or so promoting a charity organization (which I can't remember for the life of me) and he visited/hung out at my coach's gym a couple of times during his stay. Archie was kind enough to share various skills and insights, such as this subject, with all of us at the gym. When I look back, I can kick myself because I didn't fully appreciate who he was at the time and didn't take advantage of picking his brains as much as I should/could have. The man was an encyclopedia of knowledge and skill. Can you believe, I didn't even know the man held the record (and still does) for the most knockouts ever in the modern era (121 KOs). Man, I wish I would have picked his brains more. Anyway, even in his advanced age, the man was very articulate and could still bang. One thing that held me back (aside from ignorance) from conversing with him more than I should have is that I was a through-and-through Ali fanatic at the time and was a bit taken aback when Archie criticized Ali's style among other things. I actually had the nerve to razz Archie a bit about his KO loss to Ali. Thankfully, Archie was cool about it and didn't take a high school kid's ramblings seriously. Archie said that if he were in his prime, he would have beaten Ali. I, of course, didn't agree and said the prime Ali of 1966/67 would have beaten him and any other heavyweight in their prime from any era, past, present, or future. Anyway, I'm digressing big time. Sorry 'bout that, just reminiscing...

Anyway, take care and have a good week.

Best Regards,
Lito