LOL yeah, I know you are right...just can't find a good answer...so do they have this button to deploy energy whenever they want?
According to this artcile they don't.
http://en.f1i.com/magazine/52167-how-does-ers-deployment-work.html
"...How are these megajoules used on track? Basically, the energy is transferred to the drive shaft in chosen straights for a handful of seconds. Ahead of the race, teams pre-select the circuit area where the MGU-K will be activated and for how long. Their decisions are based on a mathematical analysis, as
Mercedes engine boss Andy Cowell recently explained during a media event held in Brixworth.
“You end up with graphs where there are ten different straights around the circuit and you haven’t got an infinite amount of energy to deploy through the MGU-K, so you do need to work out on which straight you’re going to deploy the MGU-K for two seconds and on which straight you’re going to deploy it for only one second.
“So we do the analysis looking at the ten straights and we can see that [on some] we are getting good lap time benefit for even longer deployment. As opposed to [another] straight where there’s just no point. For example we’ll select a 0.2s MGU-K burst and then turn it off, save the energy for another straight where we’ll get a greater reward on the lap time.”