Most Reiki practitioners allow the energy of Reiki to flow through their hands and into the patient they are working on at the time. It is an attachment to the Universal Life Force that they are accessing to bring this healing power to people. As the practitioner goes through the many attunements for each level of Reiki, they will also learn the symbols that are drawn and repeated for maximum healing. Each level has its own symbols.
Each attunement received by the student raises them to a higher frequency. The attunement can only be given by a Reiki Master because their connection to the Reiki frequency is so high that they can help you connect to it as well. An well known example is tuning in a radio station. You have listened to one all your life, but now want to go to another new higher one. Such is Reiki, you are just moving from one vibrational frequency to the next.
As this happens, you become more aware and more attuned to may things. You will start to notice it a little at a time. Maybe you are more empathetic or maybe you can attract wildlife wherever you go. You will definitely notice a change as you go through the Reiki levels on your journey to Reiki Master.
During an attunement, the Reiki master will write the Reiki symbols onto your aura. These attunements are permanent. Your energy frequency will then adjust to the higher vibration. These are given usually in groups of ten students. The Reiki master will approach each student, draw the symbols and anchor them in. You may feel like you are floating, you may feel like you are in a trance, you may spin somewhat, or you may feel nothing at all. Once you have access to Reiki, you can begin your journey of helping others with what you have learned.