You never really keep broadcasting on, before I started multiboxing I thought it worked how a lot of people here thinks it works: main character does something and all the others do it too but that’s not exactly how it works lol. I tried having broadcasting on when I first played but the problem is all your windows won’t be the same so your characters will run off in different directions 
There’s many different kinds of binds you can setup but mostly you’ll be using “assist target” (also in Keybindings/Targeting in game) so I cast frostbolt at some mob and the other characters on my team assist me and cast at my target whatever button I pressed but for their client. You could do it with broadcasting and get the same effect though assuming you don’t have anything unique setup and it’s just 5 of the same spec casting the same spell.