Originally Posted by yellit

Sounds like you have it figured out....
Another way to think of the process is to think of a 2 stroke port fed engine piston that is coming up on the power stroke as the piston wall has just sealed the cylinder intake port on its way up the cylinder.....

Pretend the exhaust port does not exist for this description......

If the spark plug was taken out and the piston was at the bottom of its travel and air was blown in the cylinder intake port......Air would come out of the spark plug hole......

As the pison starts upward (crank is rotated)...air will continue to exit the spark plug hole as long as it is still being blown in the intake port........

Until....the piston covers the intake port on its upward journey.......
At that instant....No more air out of spark plug hole and beginning of delivery or pressurization......

If the spark plug were in place ...you could not know when the piston covers the intake port as no air could flow through the cylinder......

This is why the delivery valve innards are removed for this test....so fuel can flow from the hand pump into the IP chamber...into the plunger barrel ...up the fuel element and out of the drip tube......until the plunger comes up and stops the flow....this fuel stop is determined to be best at 24 BTDC.....hope this rambling makes some sense....