The entire system flows naturally within and through an attuned Practitioner based on one thing -- Life Force, also known as Qi or Ki. However, in practice and philosophy, there are guidelines - Reiki Principles - meant to simultaneously focus and open the Practitioner's mindset. There is the focused awareness and the open attitude, both similar to mindfulness practice, which allow the smooth & clear flow of energy through the Practitioner.
Reiki Attunements do several things. They clear and align the Practitioner with the flow of universal Life Force, and from that naturally flows healing within the Practitioner. It is this, then, which strengthens the Practitioner's ability to extend the channel of flow into the world around him/her, to send energy to others, to animals, to plants, etc.
All of this invariably changes, transforms, and grows the Practitioner - sometimes rapidly, sometimes over time, sometimes in "growth spurts". And, should a Practitioner not actually practice - as in, not intentionally sitting to a self-Reiki session or consciously sending Reiki to another during grief or illness - the "channel" opened by the Attunements remains and the internal transformations still take place however slowly and almost imperceptibly.
One of the most important things a Reiki Practitioner or student or Attuned individual can do is to recite, even just mentally, at least once a day, the Reiki Principles. I learned them as follows, and attribute them to the teachings of Dr. Mikao Usui, "Just for today, I will let go of anger. I will let go of worry. I will act with kindness toward every living thing. I will earn my livelihood honestly. I will give thanks for my many blessings."
They are simple and powerful affirmative reminders of the responsibilities a Practitioner accepts when working with Life Force, and they open us to the capacities for moving fluidly and purposefully through our day - fluidly releasing emotions and purposefully behaving with benevolence. It does not presume perfection or require it or facilitate it. These principles and their recitation or affirmation merely set the intention and attention of our minds and hearts.