This one is a good example of read the manual, I would have to admit.
During cranking, press and hold the ether button for 3 full seconds. This fills a cup, which when the button is then released, this measured quantity is administered to the engine in a calibrated way. When the button is released is when you will observe the engine to fire. Hold for 3 full seconds during cranking, then release the button while still cranking. It does not activate unless the starter is engaged and the engine temp is cold. There is no fuel pressure interlock, that information is not correct.
Ether is not allowed into a warm engine because it can cause an over-RPM condition, since the governor does not have control of the engine. The system works perfectly as stock.