That's not how I'd describe Private Boolean....
I would have probably said something like...
Private Boolean (PB) is part of the set of features to allow Rules to be combined/cascaded/work together.
PB is a switch that exists in each and every Rule that can be manipulated by any Rule. However, PB must be enabled for that manipulation to do anything. 
Under Restrictions, down at the bottom, is one of these:
It is disabled (PB is ignored by this rule) by default. PB is also initialized to be True. (Which is what the [true] in the screen cap means) Therefore, enabling PB only, does nothing. It's true, and true means the Rule gets evaluated because of it's events.
When PB is enabled, and something sets it false, that Rule is shut off... as if it got deleted. Set that PB to true again and the Rule will function as intended. A Rule can manipulate it's own PB, or the PB of another Rule.
Imagine a rule that turns on a light from a Motion sensor. Now imagine you want that to work only on school days, unless you override it with a switch.
One Rule to do that is kind of hard because "school days" has to be weekdays minus all the holidays kids get (and that I don't.) So you need a rather large rule just to figure out if today is a school day. But once you have THAT rule working.. add in the override switch and it can be used to True/False the motion/light Rule's PB.
