I just to extend on the ZerO vs Flash game because its like you said "llustrates beautifully how you can use this principle at varying stages of your play to create a strategy." and i love this particular game soooo much
ZerO vs Flash on matchpoint, where Zero opens 2 hatch muta super fast, and saves up enough gas for his first first few mutas 6-10 THAN as the mutas are aheading towards flashs base, zero pulls all guys off gas(the transition step), takes two expansions the mineral only and the 3rd gas. Begins amassing drones, the mutas (with the help of the match point cliff) are not to do damage, like a normal 2 hatch muta play would need to, but rather to force turrets, and kill marines, not scvs, hes aiming to delay the first push while his now 4 base 3 gas economy kicks in. (also a note that the mutas arrive BEFORE stim! which is crazy fast) but like that game was just beautiful, with that opening than transition into a solid macro game (zero was more ahead than this build planned for because he did do quite a bit of damage with thoe early mutas, but he probably didnt need to do that much, because the transition imo is what is meant to set him into a bigger lead.
I think zero was focusing on the mid game, like he wanted those 3 gas, 4 bases while flash was on 2 base, and he found probably through countless practice games, there were 3 (im making this number up its just an example ) good ways to get into his desired midgame, maybe 12 hatch opening, 3 hatch before pool, and the 2 hatch muta (again making this up) but taking into account Flashs play style, timings, he decided that the muta play would be the most successful, at delaying flashs first push (which holy crap remember the time when he killed like multiple zergs with just the first push...that was crazy)
so while the normal 2 base muta is really aggressive, and risky in the way u NEED to do damage. ZerO's play i think is a bit different in the way its the key stepping stone to his mid game*, and not what directly causes the lead through muta damage but rather the combination of the delay the mutas gives*2 (killing marines that dont have stim let alone range) with heavy drone production/expanding (due to drones off gas) that provides him the lead he wants. Which resulted in ZerO crushing Flash.
*(the expanding was not risky, in the sense of pregame preparation he knew around~ what time the mutas would buy him, and he would use that time to expand and drone up)
*2 so T players normally want to push out while mutas are building, (which forces the mutas to not fly directly towards the terrans main), because u(as the T) dont want to move out without a big enough mm force that the mutas can just pick away at you.
So when Flash sees the mutas imo two things should cross his mind, 1st the mm push (if he was going to do one) is going to need to be bigger, otherwise a flock of mutas will pick away, and some lings with crush the rest of it. 2nd these are really fast mutas! what normally happens with 2 hatch muta? super fast mutas that are here to do alot of damage (with more reinforcing) Flash doesnt know that zero has taken guys off gas, so in his mind its okay to build extra turrets its okay to stay on two base and turtle up. Because of simply possibility of more mutas coming (maybe even with a +1 upgrade) he has to play defensive, and like you said zero takes great advantage of that (not decided in game, but outside beforehand)
Thankyou for writing the OP theres need be more community analysis :D cause its just so fun to read and learn about this stuff!!!