Muscle Energy Technique (MET) and Proprioceptive Neuromuscular Facilitation (PNF) both use your own contractions to get a muscle to let go — taking advantage of the fact that a muscle relaxes more readily immediately after it has contracted.
What you actually do
You will be asked to push against my resistance at a controlled effort — often quite gently — hold for a few seconds, then relax. The limb is then taken into its new range. Repeated a few times, range typically improves noticeably within a session.
Why use it
It is active rather than passive, so it tends to hold better than stretching alone. It is comfortable, which makes it suitable where tissue is too sensitive for deep work. And because you are contracting the muscle, it starts rebuilding control through the new range rather than just creating slack.