Mike, Breathing into the sides takes all the slack out of the
tissues laterally, which exposes any tension in your lower back–if it exists.
So breathing is just engaging the tension already present inside of your body.
Therefore, either you back off with your effort or you practise breathing exercises while sitting
or even lying down to allow you to release the tension that is brought to the
surface via breathing practice.
It will take time–not just a few practice sessions–but
likely some months or maybe even years to fully release the tension in your body
and open up your breathing without force. The aim of this programme is to give
you an overview in the first 10 weeks and then you go back and take whatever
time is necessary for you personally to complete the programme properly.
Good luck!