Negative selection in T cell development is often simply described as preventing effector T cells from recognizing self-antigens. This is complicated by regulatory T cells developing from T cells with moderate affinity for self-antigens. If recognition of self is useful for immune tolerance, why don't T-cells with strong affinity for self-antigens also become regulatory T cells?
The selection is based upon the strength of the binding. A strong bond causes a signal in the T-cell to self destruct (apoptosis).
Wikipedia has a post on T cells. Perhaps the moderate bonding become regulatory cells, because these cells have to recognize those T cells that survive negative selection, which are most likely to cause autoimmune responses over a long period of time.
Strong affinity to self means the immune response will become a danger to the host.