Imagine a counter with a display consisting of seven digits with a decimal point after the fifth digit. e.g. 00000.00 (a bit like a sports stopwatch display).
Every time the input to this counter gets a pulse, it increments by 0.01 e.g. increment it 137 times and the display will read 0001.37
There is also an oscilator which gives 100,000 pulses per second. If it is connected to the counter directly, the count will count up in milliseconds. i.e. after 1 second it will read 1000.00, after 2 seconds it will read 2000.00, etc.
Now if I ad a circuit which only allows the counter to run when there is light on the photodiode then it will only count when the shutter is open and light is falling on the diode.
We then have a simple conversion to do to work out shutter speed:
1 second = 1000.00mS
1/2 second = 0500.00ms
1/4 second = 0250.00ms
1/8 second = 0125.00ms
and so on.
You compare your actual shutter speed with the theoretical speed in milliseconds to see how close you are.
The final part is a reset switch which sets the counter back to 000.00 just before you fire the shutter being tested.