I’ve seem tons of explanations of how the shutter works. This is particularly interesting when taking about flash photography when the so-called x-sync speed is the cutting limit of flash usage unless High speed sync (or similar techniques, like hyper-sync are used).
This youtube film helps understanding why things work the way they do. Pay attention to the recording of the shutter at 1/1000 and notice the narrow slit between the two shutter curtains. If you were to use normal flash with the shutter in that state, you would only record the light that would pass through that slit and result in a bright band in a black frame.

Here’s the high speed shutter recording (youtube):