It is true that no two Reiki sessions are alike. I have been learning, practicing and teaching Reiki since 2012 and am still continually surprised. What will happen cannot be predetermined. The short-term single session benefit can be relaxation, calming, there maybe reduction of pain in some area, or the client may receive an insight. The sensations in the body can be relaxing, heat, cold, tingling, floating, sleeping or zoning out. I had received a single-session handson reiki in 2013, and that stopped my dizziness for a couple of years. Still surprises me, when I think about it!
In emergency states - it can be helpful - my personal experiences is a night-time loose-motions upset for my companion on a train, reiki stopped and calmed it down, enough to reach a doctor at the destination. Another emergency situation was a visit to the hospital, and finding a needle spot for collapsed veins with the support of distance reiki.
However the best and most cherished by me are the multiple-reiki sessions. This is a true and underestimated power house. It can enhance your spiritual journey, support reduction of unwanted habits, and reduction and relief from chronic ailments. This is a true story: my friend's mother has a chronic bladder issue. After about 4-5 months of daily, distance reiki, her condition improved so much, that she stopped wearing diapers. I still cannot get over this one!!