I'm refering to the equations for S11 in the attached picture. If V2+ is equal to 0, I would think that this is as if port 2 is short circuited, i.e. the load is 0 ohms, not 50 ohms. How could it be anything else? I've been flipping through the textbook for the last 5 hours trying to figure out what I'm missing, but no progress.

 

For reference, here's page 2 of the example.

 

I think you are confusing V2+, which equals zero, with V2 which is not zero. V2+ signifies the voltage of the wave from outside the network going into port 2. Since there is no generator on port 2 and since there is no voltage reflected back from the load into port 2 - because it is a matched load, there is no wave going into port 2. Hence V2+ =0. The only signal at port 2 is that which was transmitted from port 1 and coming out of port 2, signified as V2-.

So V2 = V2- = V1+ times s21: the transmission coefficient of port 1 to port2.