Basics of AIS: First of all what makes this method different is that the client is ACTIVE. The client is working constantly throughout the session. That means that the client, not the therapist, moves their own body as the stretches are performed. The therapist assists the client only at the end of the movement to enhance the stretching process.
This modality ISOLATES the various segments of the muscle depending on the stretch being performed. Aaron Mattes developed these stretches so that you could isolate the distal half of the muscle-tendon unit from the proximal half of the muscle-tendon unit. This means that you can stretch them separately and you can assess which part of the muscle-tendon unit is not flexible. He also created a method for stretching different fibers of the muscle depending on the angle of the stretch.
So the STRETCHING and strengthening portions of this method have the client active at all times and aim at identifying and working with different aspects of the muscle-tendon unit.