Felt heaviness for 1st time . . . Want more!

[bts]

....................
Friday, as another experiment with my driver, took the club away very briskly with what felt like a 1/3 backstroke, then changed direction very fast and proceeded as usual.

Congratulations! That's one way of "loading the lag", "interrupt it in the middle of the backswing with the downswing". You'll do it more elegantly, instead of "very fast", once you getting used to it.

The club then felt very heavy and it felt as though I had to strain to get the clubhead (my feel) to the ball.

Congratulations, again! You successfully "sustain the lag", by keeping accelerating (straining) it, which also keeps bending/stressing/strengthening the shaft.

This drive outdistanced my buddy by 20 yards (normally he is 20 yards longer).

Sure it did! You not only increased the "effective weight" of the clubhead (or the strength of the shaft), but also make it smoothly fast by "sustaining/accelerating that weight (or shaft flex)" through impact.

Did I feel the elusive heaviness? Is that deliberate, quick change of direction important? Any further suggestions for someone who has always thrown and probably flipped the clubhead?

It's not elusive.

As long as you can harvest it and try to keep it, again, you'll do it more deliberately, yet less "in a hurry" (that heaviness/resistance keeps you from being in a hurry), once you get more familiar with it.

BTW, take a shot of your swing next time, you'll find all those "imperatives and essentials" in it (with merely the intent of "loading it and sustaining it").

[12 piece bucket]

I was all hot and bothered when I read the title of this here thread. I thought it was going to be about your significant other.