What is Attunement?
Everyone is born with Reiki energy but cannot use it consciously at will. Attunement is a process, which activates the ability to transmit and use Reiki. It is to be done personally by a Reiki teacher through a process called "attunement". The process is also called empowerment or initiation and is the most important part of Reiki learning/training, which must be done by a living Reiki teacher. It is not possible to become a Reiki healer without receiving the attunement.
Attunement is a ritual and an act performed by the Reiki teacher that creates a healer. The process does not require a mysterious atmosphere. It is done in a special manner to activate the energy channels and allow Reiki energy to flow through the recipient's energy system and adjust it so that it can carry the Reiki energy. The process is something like "fine-tuning" the physical and the non-physical bodies to a higher level of vibration. As soon as the attunement is complete, the recipient becomes a Reiki healer and can begin giving healing sessions.
Attunement is not a healing process. Do not expect to be cured of all your diseases just through attunement. It is only meant to activate the ability to heal. After receiving the attunement, you have to use Reiki regularly to cure yourself or others.
Every Reiki teacher has his/her own way of passing the attunements. One thing is common however, that no equipment or machine is used to pass the attunement. Some teachers do it collectively while some attune every student individually. I do it individually.
Reiki attunements are permanent in nature, for life time. Once a person is attuned, he/she remains a healer for the rest of the life. No body, not even your teacher can take it back. The attunements are NOT lost due to general anesthesia or any other health issue.
There are different attunements for each Reiki level.