Sometimes it is just a switch, but on ours it's opening/closing a series of valves buried within all the pipework, and not on the boiler itself at all.
It's one of those simple things that plumbers (mostly men) like to keep to themselves to make them feel superior and ensure that they're wanted, so they can keep coming out to charge for simple things, yet they make a big deal out of it, make it sound/look harder than it really is, etc.
YouTube is absolutely brilliant for showing just how simple things can be and improving the confidence/capabilities of "normal" people.
After years of suffering with successions of "dodgy" tradespeople (even those from larger/proper firms), OH started doing loads of things himself, either by trial and error, or watching lots of YouTube videos, and we barely need tradesmen anymore, especially now OH has build up a garage load of tools and spare components.
Having seen how much he can now do, I started following his lead and doing loads of things myself too, again by watching YouTube, etc. I've just replaced the old manky shower screen which a brand shiny new glass one, which involved drilling new holes in the tile covered walls, levelling it, setting it at exactly the right height for the rubber seal at the bottom, sourcing screw caps to match, etc. The sense of achievement is absolutely amazing, even moreso that the last time it was replaced, we had a "professional" plumber who charged £250 for labour only and it wasn't even level!!