A normal piston ring is like what's used in a car or lawn mower engine. Roughly square in cross section and it relies on it's tension against the liner to make a good seal. Always fitted in a groove slightly below the top of the piston (obviously ).

A Dykes ring is L shaped and fits in a groove at the very top of the piston. This leaves part of the ring open above the piston so that any pressure in the cylinder can immediately get trapped behind the ring and force it out against the cylinder liner. The idea behind this is that the higher the pressure (as in combustion) the harder it gets pushed against the liner for a better seal. Turning them by hand doesn't generate much compression pressure and what there is can leak out fairly quickly. That's why they can appear to be worn out but in fact the ring hasn't had a chance to seal the way it's supposed to. Sometimes a starter is necessary to spin it over fast enough to reduce the leak down time.