oh heh sorry... just habit to type the abbreviation
hg = head gasket
block test is like a rocket shaped tube with a rubber bung on the end, you fill it with blue fluid that is supplied with the kit and then put it in the top of the radiator where the cap would normally be
you then hold it in there till your thermostat opens and the water circulates... if the blue turns yellow or green you have exhaust gasses in your water