Radiation is routinely used to treat cancer. Cancer cells have less resistance to radiation than normal cells. The debate is whether it makes sense to accept the risk of radiation to healthy tissue. High level radiation certainly is associated with a cancer risk to healthy tissue. One could draw a straight line from that risk to zero-zero, and that is what is routinely done. The proponents of low level radiation assert that rather than use a straight line, they should draw their straight line to zero-less than zero, so some low level of radiation is associated with positive benefits for healthy tissue, rather than merely act as a treatment for small cancers that you don’t know about yet. I don’t know what a mechanism for such a health benefit would be.
The mechanism is theorized by some to be that the radiation stimulates the immune response, exercising and strengthening the mechanisms which manage the ordinary tissue damage that occurs every moment of every day due to oxidants, other natural metabolic waste products, and natural cell death.