Yoga posture adjustments can be transformative for the yoga practitioner. When the teacher gently shifts the student’s knee, or puts a steady pressure on the lower lumbar spine, the student can experience a feeling of being cared for, or being guided into a place that dings like an ‘a-ha’ moment. On the flip side, too much touching and adjusting from the teacher can be triggering and have the reverse effect of making the student feel like he is not doing any of the poses to the teacher’s standard. Or, the teacher is too hyper-vigilant and over-assisting her students.
Yin Yoga teacher, Bernie Clark says "as the teacher--your job is not to impose alignment on somebody else; but to give them clues along the way, give them basic stuff (like, stay in the right lane, turn left, turn right...some simple fundamentals), but mostly you are trying to help them find (their) alignment, so that their practice becomes safe and intuitive."
In her book “Yoga Posture Adjustments and Assisting – An Insightful Guide for Yoga Teachers and Students”, Stephanie Pappas offers three important points for yoga teachers with regards to assisting students: The first point is to “use caution and compassion when working with someone else’s body. Everyone’s body is unique.” Secondly, to “move slowly, carefully, and consciously when adjusting someone” and thirdly, “ask for feedback and heed your [student’s] requests to stop or release the adjustments. Never force someone into a posture." This is valuable information to take into consideration and in taking from both Bernie and Stephanie, there is also a fine line between sufficient assisting, and also allowing the student to find their own sense of alignment, and one that is comfortable for their particular body structure, as well as allowing the student to feel safe and free from injuries.
We live in a day and age where it necessary to respect other people’s boundaries. If you are a yoga teacher, it does not automatically give you full permission to touch your students without their permission. Before starting a class, a teacher could offer cues for those who wish not to be touched, such as raising a hand for yes to being touched. In some yoga studios, there are ‘yes’ or ‘no’ chips that students can place beside their mats, and flip them during class according to their level of comfort.
It can also be recommended that the student turn under a corner of their mat if they wish not to be touched or assisted during the class. Quite often there are students who may be experiencing trauma or PTSD, and any kind of touch from another human when they are not expecting it can throw them off center. Letting the student know by gently approaching them and asking them if they would like to be assisted would be a sure way not to cross any boundaries.
If the teacher has permission, how she assists her student is very important. She will want to make sure that she herself is grounded and steady first before reaching out to help her student. The position of the teacher’s feet and a whole hand contact will allow the student to feel safe and steady. The teacher should be sure that she is accurate with her cues to help prevent injuries. It is one thing to be precise, and quite another to be accurate! What works for one student, is not going to necessarily work for the next, and so the same, precise assisting techniques used over and over could lead to trouble. Accuracy, based on the student’s body type and even their emotional state, moment by moment, is what the teacher should indeed be paying attention to.
The teacher must also be aware of how long she assists the student. Between three to five deep breaths is usually sufficient, and developing the awareness of the student’s body sinking deeper into the pose, or moving away from the teacher’s hands, giving the cue to release. When taking the hands away, the teacher should do so slowly, making sure the student is steady and won’t lose balance. The teacher should also be aware of her own body comfort during an assist.
Getting creative with yoga assists can be a fun way to connect with a student if they are willing. For example, in the Seated Forward Fold (or Caterpillar in Yin Yoga), the teacher may choose to perform a ‘back-to-back’ assist. The teacher would place her back against the student’s back, her feet and hands firmly on the floor, and then laying her head back on top of the students head, gazing up at the ceiling. She would gently ask her student to relax and deepen his breath as she leans her weight into him.
John Fedderson, a Yoga teacher from Devalila Yoga (where Stephanie Pappas is director), said “there is nothing worse than a half-hearted assist. Don’t rush. Take an extra breath and use conscious positioning to create a synergistic effect between you and the other person.” 
Proper alignment will come to the student with time, especially with the teacher’s assistance. Erich Schiffmann of Freedom Style Yoga, realized in practicing Iyengar Yoga that the practice was not about putting your body into a specific shape; rather it is about "looking for the places that ding...so that the alignment comes from within."  This is the eventual goal of the student, and of course it comes from a place of feeling secure in the capable, accurate and compassionate hands of their teacher.
1 – Bernie Clark, Yin Yoga teacher http://www.yinyoga.com/newsletter40_alignment.php
2 – Stephanie Pappas, ‘Yoga Posture Adjustments and Assisting – An Insightful Guide for Yoga Teachers and Students’, Trafford Publishing, Page 3
3 – John Fedderson quote from Stephanie Pappa’s book, ‘Yoga Posture Adjustments and Assisting – An Insightful Guide for Yoga Teachers and Students’, Trafford Publishing, Page 126
4 – Erich Schiffmann of Freedom Style Yoga www.freedomstyleyoga.com Quote taken from Bernie Clark’s article: http://www.yinyoga.com/newsletter40_alignment.php