There are more
possible explanations than could be fit in an HN comment. What are you looking for?
I think Reiki would say that "energy" assists you to heal your own back, but it doesn't specify how that healing takes place. In addition, I think Reiki distinguishes between the "energy" of the practitioner, and "energy" of the universe which flows via the practitioner, and ideally the latter assists you.
If we assume (for the sake of this discussion) the relief of tension in your back takes place and you are then looking for a mechanism, I can immediately think of numerous plausible explanations we could begin to explore, some of which we could measure, and others are beyond our capabilities to measure.
I'll give some examples of plausible:
- The tension is caused by signals in your neural system. The Reiki experience caused a change of data in your neural system due an information "covert channel" that exists between your system and the practioner, and your muscle tension is relieved.
- The tension is caused by chemistry in your muscle cells or intercellular matrix. The Reiki experience caused a change of gene expression due numerous hormonal changes that occur in your body when you are in that environment.
With both of those examples we have scientific study methods for looking into more details. E.g. for the first, it would involve looking at neural signals and the practitoner's neural signals and other factors such as position, timing etc. to detect hidden signal transfer. For the second we could monitor all the hormones we might think are relevant, or if we want to search more broadly than just markers we already know about, perhaps monitor some general molecular distribution signals such as chromatography or spectrometry.
There are numerous modern methods for looking for signal relationship, but also numerous confounding factors.
The list of plausible things to explore is vast though, so ruling out one or two doesn't rule out there being a mechanism. However as you explore mechanisms, if you find something that shows a signal relationship, then you can zoom in and look into plausible underlying mechanisms for that mechanism, and repeat. There is no bottom, but the more levels deep a signal is shown, the more people are likely to call it a mechanism rather than an effect.