Positive Seal
There are several types of O-Rings. What it does, in a general sort of way, is make a copper ring around the top of your cylinder. This little copper wire sticks up just a LITTLE bit above the deck of the block.
Now, depending on how you do it, this ring seals your combustion chamber by making contact with the head. Metal-to-Metal clamped down with such a small crosss-section of area in contact with the head, the contact pressure is enormous. Basically you make the block and head as one.
Some other methods let you use the standard headgasket on top, and the aame effect is achieved, with the O-Ring biting into the sealing ring of the head-gasket and clinching it tightly between the head and hte block.
Either way, when you run high BMEP in an engine, the head gasket likes to not hold in all the combustion gasses. If you detonate, the head gasket will blow and away goes your coolant, and whatever else necessary....
Basically the advantages is that you don't blow your head gasket every six months from ruunning high-boost, high-HP through a block designed to live at much lower stresses.