Quite a common mod in extremely high-boost or competitive applications when the cylinder pressures are high and exceed what the head gasket can withstand. You basically cut a relief groove in the block (or head) and fit a small piece of wire (or something that forms a 'ring', as it's also known as "O-ringing the block") around each of the bores. This helps the head gasket (which usually is a copper one or a performance gasket that's designed to accept rings) clamp firmly against the mating surfaces, giving greater sealing and preventing blow by from either the combustion chamber or waterways.
As mentioned, you should only really use it with copper head gaskets or those that are specifically designed to recieve the wire that is laying on the surface - otherwise you end up with a protrusion from the face which doesn't help sealing, quite the opposite. You can sometimes use standard gaskets but an awful lot of care must be taken to ensure the ring does not protrude too much, or cut into the gasket.
You can fit the o-rings to the head but that is a high-precision affair and can be quite costly if it goes wrong (i.e. it's easy to centre a cutting tool around the bore, not so the surface of the combustion chamber - and a block is easier to replace than a head that's had a lot of work). Depending on the size of rings and the desired clamping forces, you sometimes must cut 'reciever' grooves into the head that match those in the block, to prevent (again) the rings cutting the gasket.
A machined, flat block surface (that's at the proper angle), a good finished head and a "decent" head gasket, however, should give you no grief - so unless you intend on running vast amounts of boost through one, or are just looking to experiment with a disposable engine, I wouldn't really worry about it.....
One downside with this process is that as you can now sustain much higher combustion chamber pressures, when you blow it up, you tend to take the head, valves, bottom end or pistons out - instead of just the gasket.........so it can be a double-edged sword
