I'm quite a noob aswell on this but what i do know is this: In Isboxer you have 2 steps for a keybind that has a spell under it which will assist your mainchar. 1. Do FTL assist me in which you set the target. I asume the ISboxer addon then sees what target the 'main' character has and tells your slave that that's their target aswell. step 2 is to actually broadcast the key.

There are several other addons that also react to the target from a boss for example. Instead of having the focus on boss X it's now on the character you're playing.