Multifuse devices, made from a conductive polymer, exhibit this PTC effect because of their unique construction.  Polymers are insulators by nature, but carbon mixed into the polymer matrix results in high conductive properties. The carbon allows current to flow through the device giving it a low resistance in its normal state. However, an over-current surge causes the polymer PTC to heat due to I2R losses causing the device to change phase from low resistance to high resistance as the carbon chains start to separate. The PTC will stay in the high resistive state as long as there is a fault.  Once this fault is removed and the power has been cycled, the PTC will return to the low resistive state.