I often have clients with depression as part of why they are coming to me. So my understanding of depression is that you are worrying about the past. About what you no longer can change. Yes there are some chemical imbalances that do cause depression. With reiki being energy it goes where it needs to go. So it can help your heal your past, present, and future. Reiki healing sessions help you relax into the here and now. Then from there you can work through the problems or events that you are remembering or are depressed over.
I create a safe space for all my reiki clients no matter what you are going through. I had a client that was depressed over her marriage coming to an end. I reminded her that she is safe and there is no judgement in the space. I told her to give everything voice to clear it out and to release that pent up emotions. She was emotional and talked the hole sesion. I listened and continued to do reiki. I call this talk therapy and I use it often in my reiki practice. She told me after that she felt lighter and more at peace with her decision to leave her abusive husband. She comes back every now and then for rreiki sessions.
Each person experiences reiki differently. So what you get out of it is uniquely your own experience.