The paper analyzes the mechanisms of the individualmachine and also its advantages in TSA. Based on the criticalmachine monitoring of the original system trajectory, it is clarified that the individual-machine strictly follows the machine paradigms. These strict followings of the paradigms bring the two advantages of the individual-machine method in TSA: (i) the individual-machine trajectory stability is characterized precisely, and (ii) the individualmachine trajectory variance is depicted clearly at IMPP. The two advantages are fully reflected in the precise definitions of individualmachine based transient stability concepts. In particular, the critical machine swing is clearly depicted through the IDSP or IDLP of the machine, the critical stability of the system is strictly defined as the critical stability of the most-severely disturbed machine, and the individual-machine potential energy surface is also precisely modeled through the IMPE of the machine. Simulation results show the effectiveness of the individual-machine in TSA.