This phrase applies to so much of the work we do together, some obvious and some less so. 
Starting  with the obvious, trauma, in some way or another, is always relational.  Whether that's being harmed by someone you thought you could trust or  not being able to enjoy physical contact the same way after your  concussion; there's some element of the way you relate that is crucial  to the healing process. 
A lot of good has come from the  self-healing movement, but the reality is wounds that were created  inside a relationship need to be worked on in relationship. 
We  can do all the meditations and journal prompts, but something else  happens once we are in that moment with someone else and an old survival  pattern activates. It takes practice to chip away at the pattern and  make something new. 
This also applies to the body and  movement. The brain doesn't understand individual parts the way our  intellects do. While one body part may be the painful one, it's part of a  larger pattern of movement – and it's in that part's relationship to  the pattern that's the cause of your pain. 
If we just go in  there and do something to your knee, but don't show your knee a new  relationship to your hip, low back, foot, etc. the likelihood that your  body is going to know what to do with that new knee is low.
Research has continued to show that the method rarely matters compared to the relationship you have with your therapist. 
That doesn't mean that methods don't matter, but if there's no relationship, it doesn't matter.