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Like david stated...engine compression leaking into the cooling system. Cooling systems are designed to operate somewhere around 7-12 (give or take exact numbers don't really matter for explanation purposes)pounds of pressure. The pressure created by the combustion process is much much greater than that. If the head gasket is comprimised in the area around the cooling passages, high pressure always goes towards low pressure. The pressure created in the cooling system is then higher than designed, the relief feature of the radiator cap releases, pushing coolant along with the excess air out through the overflow tube. An easy method of checking this is to take off the radiator cap on a cold running engine. Watch for bubbles forming in the coolant, the amount and size of the bubble will indicate the severity of a leak. Also, after the engine has been shut down....coolant will also go into the combustion chamber of the affected cylinder if the cooling system is kept closed. A major leak will leave coolant, a minor one the coolant will evaporate.
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