I had the intention more than 2 years ago to look into flex downtimes, to enrich my workaround for recurring downtimes for raw users, but I never got to it. )-:
So I never used the feature, but what I assume from image in docs (Scheduled downtimes) is that when host / service goes down / critical within set normal downtime period, it will start from that moment the flexible downtime.
So if in this example image the host goes down (hard) at 15:50, the flexible downtime is started and makes the downtime extend till 16:20 (30 minutes later).
And if the host goes down at 14:10 the downtime will stop at 14:40.
But again, just assuming here … (-;