Your body utilizes Vital Fluids, which is a modification of the Universal Fluids that make up everything in the Universe, that are altered to fit your own spirit and body. Vital Fluid can be thought as an energy force that maintains the various Force centers, otherwise known as Chakras, which are responsible for keeping all of your bodily functions in harmony. The more Vital Fluids you have, the healthier and more vitality you will possess.
This transfer of Vital Fluids is done with the assistance of two beings. One is the loving and caring spirit who uses the physical person giving the passes as a channel to supply you with energy. The person giving the passes must be in harmony and full of goodwill toward their fellow beings. As the picture illustrates, the spirit stands behind the pass giver who stands in front of you to deliver the vital fluids.
I have received passes and I have definitely felt better afterwards. Could this just be all in my own mind and I wanted to feel better? Yes, that could be one explanation, but with the other phenomena I have witnessed with Spiritism, I am a believer.