[PD] Extreme Newbie Question: Flow Order?

[Ryan Supak]

Hi, I'm brand-new with PD.  I'm on Tutorial "03.connections", and I'm a
firm believer in not advancing to the next level of complexity until you
fully understand the level you're on.  So....
 
The first bit of text in the Tutorial says that input to the rightmost
inlet causes the function do be done.  However, what follows is an
example of an addition object that only executes when the leftmost inlet
receives input.  (The rightmost input acts as an argument for the
operation in this case.)
 
So, my question is this:  is there something special about the "+"
object that makes it an exception to the "rightmost" rule all the time,
or is there something special about the way it's set up that makes it an
exception to the rule?
 
Thanks in advance for helping a novice!
rs