My citation is the contract governing how I am paid. Pay time begins when all exterior doors are closed and the parking brake is released. Pay time ends when the first exterior door is opened (our contract says the it should only be the main cabin door, but in reality the clock stops as soon as the rampers pop the bag door).
And yes, pay is by the hour. The excuse of the airlines pushing for these pay rules is that our hourly rate is high enough that it covers all of that stuff too. They'd be happy to switch to pay by the duty day if we in turn halved our hourly rates. Just to put it in perspective, On a busy day I may be on duty 12-16 hours and only paid for 6-8 hours. The shortest possible duty day is a single flight day which will have a minimum of 1:00 of time I am "on duty" but not being paid, and that does not include getting through TSA which I am supposed to do before my duty day starts.