pin #5 is the solenoid valve. To my understanding the code you are referring to allows the push button (startpin) to open the solenoid valve (pin 5) to allow water to flow to the flow sensor.
The detach interrupt is in place while calculating flow rate and sending data to host. It is enabled again after the data has been sent.
Interesting, I was viewing this as running vs. stopped and looking for the something to transition from stopped to running. I really appreciate the feedback, but I'm not quite how to translate that into this code. If there are any good examples or other threads that you think I may benefit from reading, please let me know!