Sentence:  When the incident shock wave is at position “a” the second <WSD>triple point</WSD>, T, is still inside the zone where the vibrational degree of freedom has not yet reached its equilibrium and the dissociational degree of freedom is still frozen at that position, as its excitation can be assumed to start only after vibrational equilibrium is reached. 
Meaning of the word marked with <WSD> tags:  A point where the incident shock wave, the reflected shock wave, the Mach stem, and one slipstream meet.