Proprioceptive Neuromuscular Facilitation

This technique brings one's awareness to the activation and relaxation of a target muscle group and may include activation of the antagonist muscle group as well.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3588663/

According to research, not much has been clarified as to what mechanisms in the body contribute to the positive benefits of PNF utilization.  The article above mentions four primary principles related to muscle relaxation, stretching, yet still does not shed much light.

PNF stretching can be utilized in a yogic practice.  Many asanas can allow the practitioner to follow the process of Stretch - Contract - Relax (or conversely, Stretch - Contract - Antagonist Contract - Relax) to create increased ROM.  

Examples: Triangle pose / Trikonasana - front leg, the practitioner can enter the pose, enjoy hamstring stretch / adductor stretch, activate those muscles and then relax deeper into the pose (possible addition of quadriceps activation after hamstring / adductor contraction); Malasana / Garland Pose – squeezing thigh adductors; Uttanasana – squeezing hamstrings, etc.

*It should be noted that the literature shows that muscle performance decreases if PNF is used before exercise, yet has the opposite effect post-exercise.  Also, maximal voluntary isometric contraction is suggested to occur between 3 - 10 seconds (one slow inhale and exhale).  Longer contractions can result in muscle/tendon injury. 

I would suggest, then, incorporating PNF instruction towards the latter half of a class, especially if you are trying to target increased ROM and soft tissue length (i.e. hamstrings, hip flexors, pectorals, spine extensors - the common shortened areas of sedentary populations).  Try 3 sets of 6-second contractions/releases in each chosen pose.