Complex sentence: They have a valve that is driven on/off the seat by either expansion/contraction caused by temperature differ from mechanical traps in that their design requires them to hold back some condensate waiting for it to cool sufficiently to allow the valve to open.
Split into simpler sentences: They have a valve that is driven on/off the seat by either expansion/contraction caused by temperature change. They differ from mechanical traps in that their design requires them to hold back some condensate waiting for it to cool sufficiently to allow the valve to open.