The phrase "to feel the stretch" means experiencing a pulling sensation in your muscles when you are stretching them.
Sentences with «to feel the stretch»
If you don't feel any stretch in the top position, you can elevate your legs. (allthingsstrength.com)
Bring your right hand down to the ground, and extend your leg squeezing your butt, so you can feel the stretch through the front of the hip. (health.com)
It is likely the therapist will feel this before the patient feels a stretch on the calf muscles. (sportsinjuryclinic.net)