Different people have given different explanations about how Reiki works. It can be defined as a divine energy, healing energy, spiritual energy, life force etc. And different healers have different ways of implementing healing techniques.
People sometimes ask what is the most important thing in a Reiki healing session. Well, the intensity of healing depends on the intent of the healer and the desire of the one that is undergoing healing. A level 1 healer can have higher healing abilities than a Master.
It all depends on the attitude of the healer. Good healers are the ones who are positive in life and dedicatedly follow the Reiki principles.
Reiki works well even if the one undergoing healing has no faith in it. However, is has been observed that those who maintain positive attitude in life have higher chances of being permanently healed.