I'd like to understand what fast shutter speed settings, like 1/4000s, actually do.
I thought it takes a picture with a duration of 1/4000 second. For example, 1/15 leaves the sensor 'open' for 1/15s.
But apparently this is not the case with higher shutter speeds. I cannot use a speedlight with shutter speeds faster than 1/250s on my camera (a Canon EOS 90D dSLR). When I use an external flash I can use the 'high speed sync' (that's what Canon calls it at least). I've read how this works and it seems that the curtains are not fast enough to be fully open in less than 1/250s. So for higher speeds the rear curtain will start closing before the first curtain is fully opened (doesn't matter whether a speedlight is used or not).
If so, then I cannot make sense of it. It would mean that I can not take a picture faster than 1/250 anyway because the narrow line of light created by the space between the two curtains is moving from top to the bottom of the sensor for 1/250s and the actual shutter speed only determines how narrow the line is. But that is clearly not true—I've shot a few pictures of a flying arrow for example that only show the object at 1/1000 and faster.
Also, if the curtain takes 1/250s to open but the space between front and rear is too thin (1/4000 or faster) that would introduce some serios rolling shutter effect——much worse than using electronic shutter at all (on my camera I can take 1/8000 with mechanical but 1/16000 with electronic). But again, this clearly isn't true.
Could you please explain to me how it's possible to take pictures with these faster shutter speeds?