Post isometric relaxation is a type of stretching technique used for soft tissue. It is performed by you actively pushing against the therapist to activate your muscle, and then relaxing and being stretched into a further range of motion.
This technique works by increasing and decreasing your neural drive to your muscles. By actively pushing into the therapist you are increasing the neural drive to the muscle that you want to be stretched. Then when you relax, your body shuts down that drive for an instant and the therapist is able to utilize the time to take you into a further range of motion.
For example, if you have really tight hamstrings you will put your leg on the therapist's shoulder and actively press down at about 20% your strength. After about 10 seconds, you'll relax and the therapist will increase how far they are pushing your leg toward your upper body. This usually happens about 3 times until we can't get any more range of motion.