Muscle Energy Technique (MET) is a form of hands-on/manual therapy, used by physical therapists to improve mobility & flexibility.
It uses the muscles’ own energy to facilitate relaxation and lengthening of the targeted muscles.
In most cases, a gentle isometric contraction is used which then causes the relaxation of the targeted muscles via autogenic or reciprocal inhibition. These targeted muscles will then lengthen and relax.
As compared to static stretching which is a passive technique in which the therapist does all the work, MET is an active technique in which the patient is also an active participant. Source