When you set your flash in Manual mode and full power, you expect to get a full power flash, right?
Well, you would be wrong! At least part of the time.
Many of you will be surprised to learn that even with your flash in Manual Mode you will only get full power when the shutter speed is slower than about 1/125th sec!
This is because when the flash is in Manual mode and mounted on the hot shoe, it turns ON and OFF with the Sync pulse that fires it through the center terminal.
When the flash truly fires at full power, like when you hold it in your hand, it completely discharges the capacitor, and it takes longer to do this complete discharge than the time available at the flash sync speed of 4 ms (1/250th sec).
In fact, the tail of the flash pulse goes out to about 6 - 8 ms before it is completely gone. This portion of the tail is well below the half power point that is used in the specification to define the length of the full power flash. This is how they get by with saying the flash is 1/1000th sec when at full power. That's the length of time between the half-power points on the waveform.
So, when the flash is in the hot shoe, the flash fires when the Sync signal is applied as the shutter opens, and it is quenched when the Sync signal goes away as the shutter closes. This stops the complete discharge of the flash if there is any energy left in the capacitor.
Now, when the shutter is at 1/60th (17 ms), the flash has plenty of time to completely dump all the charge on its capacitor. However, at faster shutter speeds, like 1/250th (4 ms), there is still significant charge left in the capacitor when the flash is squelched.
This remaining charge is also why the flash will recycle faster when at 1/250th shutter than when at 1/60th, even when the flash is in Manual mode and set to 1/1 (full power).
This also explains why the image from the 1/60th shutter is brighter than the image from the 1/250th shutter when the flash is fired at full power. The tail that is cut off at the higher shutter speeds but present in slower shutter speeds, contributes to the exposure.