Reiju and attunements are one of the five main elements of the system of Reiki. Below is an excerpt from The Japanese Art of Reiki.
Reiju – (lit. Japanese) spiritual blessing
Description
Reiju is a ritual that is performed by a Reiki teacher on a student and is one of the five original elements of the system of Reiki. A student sits in seiza, or on a chair, while the teacher completes a physical and energetic ritual around him or her. The student practices Jôshin Kokyû Hô throughout the reiju.
By performing reiju the teacher is creating a safe space for the student to draw as much Ki as he or she needs through the body. This is executed with the intent that ultimately the student will remember his or her connection to the universe. Naturally, as the student draws this Ki, the benefits related to energetic clearing will also occur.
Within the system of Reiki students must receive a minimum number of reiju from the teacher. The more reiju a student receives – the better. In traditional circles in Japan reiju is generally given whenever a student and teacher meet and this may be as regularly as once a week. You may receive from four reiju in a Reiki Level I course, three in a Level II course and one in a Level III Master/Teacher course – but this is the bare minimum. A student’s graduation from one level to another is not a result of the reiju but, rather, it is one’s personal practice that allows one to move forward. Reiju is seen as support for the student’s journey – a blessing – rather than the crux of the entire system. The system of Reiki is so much more than reiju alone.
Reiju does not use symbols or mantras and is one complete ritual that does not alter with each level of the system. It remains the same – it is you the practitioner that changes.
Reiju does not give students any specific ability. The intention behind the rite is that one gets what is needed for one’s personal journey – nothing more. You do not ‘become attuned’ to any of the levels within the system of Reiki. Ki can be channelled by anyone at anytime and therefore cannot be given or taken away from a student. This lack of intervention on a human level evidences that no one can ever come to harm with Reiki. You are an energetic being before and after reiju – all that you are doing is giving over to receive more of your inherited blessings. The joy of this is that it supports your journey of self-responsibility; you are always in control of what is happening with Reiki even if it is in an unconscious manner. Naturally, the more human interference in the process, the more chance there is of human weaknesses coming in to play. The responsibility placed on teachers performing reiju is great as they must not be tempted to manipulate, or create more out of, this ritual. Spiritual progress is therefore a must for any teacher working with reiju.
Source
Reiju appears to originate from Tendai Buddhism. This is not surprising as Mikao Usui was a Tendai practitioner throughout his life. Reiju is said to mirror a Tendai ritual called go shimbô that is also known as Dharma for Protecting the Body. Go shimbô is a purification process and is one of the first esoteric rituals that one completes in Tendai. These esoteric teachings are passed from teacher to student and are not available to the public.