and you are welcome in advance.
let me start with this. you want to force subscription when you want to detect change soon as the change happens. like in your piston for lines 120 - 133, if you wanted that if to be checked soon as @MikePresence variable changed you would want to force subscription to that variable.
otherwise it will only be checked when the piston is next executed due to some other event subscription. which in this case would be one of the subscriptions from the IFTTT executes triggers or any of the changes to triggers.
so, if @MikePresence was a global variable that is also modified in another piston, it would not get checked in this piston till one of this other triggers, triggered the piston.
does that sort of kind of make any sense?
please dont hesitate to keep asking if that raises more questions than it cleared up.