Hi there.. ive had a bit of experience working with patients on ICU and surgical wards where the ACBT is a key part of treatment..

just to quickly (hopefuly) answer your Q -

a high lung volume = a large volume of air taken into the lungs= a deep breath in, at or above normal tidal volumes.
to the pt - "take a deep breath in"

a low lung volume = a normal/small volume of air taken into the lungs= a normal/small breath in
to the pt - "take a little breath in"

you mention youve read "a lung volume below FRC will move the EPP towards the alveoli and smaller airways" .. ive not read this myself, could you possibly post which article this was? my understanding is that just a low volume (but not necesarily lower than the FRC) is sufficient. i just tried doing the FET from below FRC and found it quite difficult myself... theres not a lot of "force" able to be generated and i dont feel the effect as much as when taking a shallow breath above FRC. Again, my experience is limited here and i sincerely welcome + hope for suggestions for relevant articles/ first hand experience on this topic to further my understanding.

My second thought is that senior physios i have worked with dont stress too much about the degree of breath in/ starting lung volume.. as the pts we work with simply get confused.

instead, i found that the following was the most sucesfull with the patients i worked with..

instruct the patient to take a reasonable deep breath in, with the focus on breathing basally and gaining lateral expansion at the lung bases and minimising and apical movements..

this was assisted through getting the pt to relax shoulders, stop gripping on with their hands etc.. and then giving proprioceptive feedback with your hands over the basal areas to facilitate and monitor their expansion..

then when they were at the peak of the breath in.. they would be instructed to do the FET as their breath out..

(without fail ive found the best way to teach the FET is to refer to it as "you know when youve got to steam up your glasses".. as this gets the correct movement usually) :P

anyways, ill stop writing now and hope that ive helped somewhat

Cheers,

Pete