" It is not possible for the flash to trigger before the blades reach the fully open position".
HuwEvans, you are very much mistaken. As Stroebel has it - "Flash-synchronized shutters are designed to close the electrical circuit to the flash at the appropriate time so that the shutter will be fully open when the flash reaches its peak light output". It is for this very reason that the flash is triggered BEFORE the blades are fully opened so that the light peak - comming later, i.e. with a delay - can reach the fully open blades (hopefully, but not always). Stroebel speaks clearly about this, even figures showing the delays are provided by him.
"Like it or not this is a fact".