Because they have a positive temperature coefficient, as their temperature increases so does their resistance. Also, as their temperature decreases, so does their resistance. PTCRs have a low resistance over a wide temperature range.
However, when they reach a certain higher temperature, their resistance greatly increases. This very high resistance can act as an open circuit and stop all current flow through a circuit. When the heat source on the PTCR is removed, its resistance will return to its initial room temperature resistance. PTCRs are made from a very pure semiconducting ceramic material.