For me as a scientist by training it is an acknowledgement that certain causes of challenges, conditions and health problems have nothing at all to do with either this physical body or this particular reality.
As this work is by its very nature intuitive, personal and sensory/feeling based then effectiveness very much depends on the skills, perceptual abilities, knowledge and understandings of the individual healer/energy worker.
It can also depend on the focus (read aims and agendas) of the subtle energies being channelled as well as the support beings and spirit guides that they work with as well as the depth of their subtle senses and inner perceptions.
In other words, there are as many levels of ability as there are healers/subtle energy workers.
You could also say that we call it ‘subtle’ energy work because we do not work with physically perceivable phenomenon or tools.
We also call it energy work because many practitioners feel / see / sense the flow of the energies through themselves and their clients as they channel. This ‘energy’ then, is not the same as that which scientists talk about or what your utility services company brings to your house. They have no relationship to joules or watts or kcal they are a description of a perception of a presence or flow movement that is non-physical.
In the same way that you can ‘appreciate’ that you can ‘feel’ the temperature increase in a room but you can not ‘personally’ see or hear or taste this change. In this respect subtle energy practitioners and healers work with a subtle energy presence and or flow that is only perceivable on an inner level. Some do this very consciously and others not at all.
For a healing or practitioner session, energies are consciously activated / invoked and perhaps channelled from one person (the practitioner or healer) to another (the client) or yourself to effect change. Depending on the energy combinations that you are connected to, then this will bring about changes on the physical, emotional, mental and or the various spiritual levels. Exactly what happens as a result in terms of effectiveness for example will be a combination of what the practitioner is currently capable of, as well as various factors to do with the client such as.
Some practitioners have very strong inner perceptions and are able to pass these insights on to their client while others have no insights and simply follow a formula approach. There is nothing wrong with following a formula approach and working blind (as it were). Reiki is an example of a system where most practitioners are working in this way. It takes time and practice to move beyond this and to start to be aware and to accept your own inner perceptions and intuitions. It takes some even more time to follow them, particularly when it conflicts with what you have accepted from your training.