How a flash freezes a moving moment
The interval when the shutter is open can be longer than the interval when the subject receives most of its light.
In a dark room, the flash interval can dominate
For a single flash within the synchronization range and sufficiently suppressed ambient light, most exposure arrives during the flash pulse. A moving hand is then recorded mainly within that short interval. The shutter display alone does not determine freezing, and high-speed sync is a different operating mode.
A shutter-and-flash timeline
Playback stretches time for explanation. It does not predict actual brightness or blur.
Do not equate t0.1 with t0.5
These measure different portions of the flash-output curve; t0.1 includes more of the tail. Compare like definitions, intended output levels and modes. More stored energy does not imply a shorter pulse, and reduced output does not shorten every flash design in the same way.
Why a sharp hand can acquire a blurred companion
Ambient illumination continues throughout the open shutter interval, adding a moving trail around the flash-frozen instant. It can be intentional or unwanted. Within sync limits, shutter time helps control ambient while flash output and aperture establish the subject exposure. The timeline separates these intervals without blurring a photograph.