To my knowledge, FM55's definition of 'hiyaw' is, precisely, that swirly sound when sustaining a note. He claims that only a small percentage of guitars (hence, 2%) really exhibit this property during bends and such.
Moreover, some people have said that the equivalent of 'hiyaw' in English is 'note bloom' (which is probably what PRS is referring to). But consider also that 'hiyaw' can be directly translated as 'cry' or 'crying' in English. As though the guitar, when performing a bluesy bend, is 'crying'.
In other words, I think that when FM55 (who is primarily a blues/rock guy) plays and listens to a guitar, he is after a guitar that really 'speaks' in a sense. And typically, you can test this out by bending and note and listening for that swirling sound.
...Which brings me back to my earlier point, maybe it's perfectly okay for guitars to not have hiyaw if your style of music doesn't even really call for hiyaw to manifest in the first place. A good example, as I mentioned, would be djent. I personally don't really care tbh, as I'm more concerned about playing cleanly over the chord changes than anything else.
Though bending notes define much of what guitar is... very few other instruments allow you such expression... I don't only bend notes 95% of the time. Hiyaw determination is a short cut to determine the other desire-able qualities of an electric guitar.
Guitars with hiyaw often also exhibit ALL of the following:
1) above average to superior sustain
2) note definition -- not muddy
3) note harmony -- notes sound 'kabit' playing chords or even double stops
4) rich overtones (most important to me)
5) loudness -- yup its not only in the winding of the pickup that determines loudness but more of the wood
6) Ability to Stay in Tune
7) Neck Stability
Woody Tone -- I mean the strings do not dominate the tone coming from the guitar; rather the body wood's timbre is heard more
9) sounds great even on a clean guitar amp
10) so easy to play even on gauge 10 strings -- makes them feel like gauge 9 or gauge 8
For me hiyaw, is a dipstick, to quickly assess guitars.