Think of a flash of lightning.
Think of the way lightning appears when it’s nighttime.
Think of the way lightning appears during the day.
The brightness of the lightning is appearing with the brightness of either daytime or nighttime.
When does the lightning appear the brightest? Daytime or nighttime?
In the nighttime scenario, all you see is the lightning and the things illuminated by the lightning during the duration of the lightning.
In the daytime scenario, depending on the the brightness of the day and the brightness of the lightning, you may not even notice the lightning.
Think of flash.
Your flash can be thought of as a flash of lightning.
Your exposure can be thought of as daytime or nighttime.
The brightness of the flash is appearing with the brightness of your exposure.
When does the flash appear the brightest? With a darker exposure or a lighter exposure?
In the darker exposure scenario, all you see is the flash and the things illuminated by the flash during the duration of the flash.
In the brighter exposure scenario, depending on the the brightness of the exposure and the brightness of the flash, you may not even notice the flash.