The condition for start up of oscillation

It seems very complex to understand the condition for start up oscillation. I found the comment in ADS tutorial: It follows from the Nyquist criterion that the presence of poles in the right-half plane and the system instability can be determined by the encirclements of point 1+j0 by the osctest generated contour S11 = Sr(jw)Sn(jw). What's the meaning for the condition?

In Ganzales book, the comment seems different.

It's another way of saying stability factor for that specific circuit is smaller than 1
k<1
This means reflection coefficient is greater than 1, where the circuit presents a negative input resistance. The wave reflected off a positive resistance load would be smaller than the incident wave. However, if the Re{ZL} < 0, then it would be possible for Γ in to be greater than 1. so, always reflected waves becomes larger than incident waves. This causes an oscillation to begin.

Best regards,