The block heater is a coil that heats up and keeps the coolant warm. By maintaining the block water in a warm condition the engine is actually starting warmer than the outside temperature. By keeping the block warm the oil also stays warm. Generally the block heater will not keep the block very warm but if it is -20C and the block is maintained at 0 C it is much warmer and therefore will start more easily. A good piece of equipment to have when it is really cold but you have to use it!