ac signal injection

Hallo,

von http://www.national.com/an/AN/ANs1889.pdf

To test a closed-loop system with an injection transformer, you must 
break the loop at some point. At that point, the reflected impedance at 
the network analyzer's A port (Figure 3) should be low, and the 
impedance reflected on the R port should be high. Those conditions are 
important, because if the closed-loop gain beyond crossover is much less 
than one, most of the signal injected by the transformer will show up on 
the R port.

For example, if the impedance reflected on the R port is only nine times 
higher than that reflected on the A port, then only one-tenth of the 
signal will show up on the A port. Nine-tenths will appear on the R 
port, regardless of the true loop response. That situation requires an 
impedance buffer (i.e., a FET probe) to raise the network analyzer's 
input impedance above its inherent 50Ω level.

Was meinen die da mit der unterschiedlichen Impedanz an Port A und B. 
Warum funktioniert es nur dann?