Probes are any external assisting devices for the beginners. Bringing the evolutionary change with such techniques by saints like BKS. Iyengar is truly safe. But the safety by modern, marketing practitioners are developing many physical ailments like scoliosis, shoulder droop, lordosis, hyperextension of knee hyperextension of elbow, having poor posture outside of the classroom and in daily activities, etc., it is the traditional texts knowledge alone can be the optimal level of stop for any physical progression.
Here is the example of the asana which resembles Cow face, “Gomukhasana”
Upper Arm: Shoulder girdle retraction for Shoulder opening. Shoulder medial rotation and adduction for fitting Humerus (arm bone) into the Glenoid fossa of Scapula (Shoulder blade).
Lower Arm: Shoulder girdle retraction and depressors for Shoulder opening. Shoulder medial rotation and adduction for fitting Humerus (arm bone) into the Glenoid fossa of Scapula (Shoulder blade).
Mechanics: To get the movement at these two joints, the student uses probes, gripping. But eventually, leave such assistance and have just touched. If the grip is by probes or fingers, the torque or tension leads to stretching of rotator cuff muscles. That is not the stable and comfortable pose according to “Patanjali Maharshi”. Slowly take the assistance as a starting step and progress to no-grip. This is how the physical effort progress towards the mental effort. Observing the pulse between two palms is Mudra or focus on eyebrow center is the Ajna chakra is Drishti as per traditional Yogic texts like Hatha Ratnavali, Hatha Yoga Pradipika, Gheranda Samhita.
So students have to progress from Grip to No-grip