From the techcrunch article:
"As that battery ages, iOS will check its responsiveness and effectiveness actively. At a point when it becomes unable to give the processor all the power it needs to hit a peakof power, the requests will be spread out over a few cycles.
Remember, benchmarks, which are artificial tests of a system’s performance levels, will look like peaks and valleys to the system, which will then trigger this effect. In other words, you’re always going to be triggering this when you run a benchmark, but you definitely will not always trigger this effect when you’re using your iPhone like normal.
Apple will continue to add this smoothing to more devices over time to avoid shutdown issues, freezing and other problems."
Ok so something that would be nice (that we won't ever get) would be some sort of notification of when iOS has hit that point where it determines when a battery is starting to crap out and the throttling begins.
As for it not always triggering the effect it just seems untrue since many are reporting the iPhones clock speed always reporting a lower frequency when the phone is at idle. My launch day iPhone 7 on 10.3.3 always is up at about 2333mhz even when the phone is idling. Seems people are reporting there iPhone 7's on iOS 11.2 are idling at a much lower frequency. So looks like the throttling is in fact happening more often and it's not only caused by only high loads such as a Geekbench test.
It seems like they are making it more aggressive as time goes on slowing down the phones even more to avoid the shutdowns.
Like I said in another thread, I'm glad I'm on 10.3.3 and at least I'll know why and when my battery is starting to crap out when the "unexpected" shutdowns begin on my phone since I have no throttling instead of it just becoming slower and slower.