Look closely between the top and second compression ring grooves and you’ll notice a very small groove. This groove is called an accumulator groove, and its job is to help to control the movement of the top ring.
It is inevitable that some combustion gases will escape past the top compression ring. This might be due to oversize ring gaps. Another reason is the unsettling of the top ring when the piston rapidly changes direction at TDC at high engine speeds. To combat this, the small additional volume in the accumulator groove reduces the pressure on the underside of the top ring.
This reduction in pressure is an effective method to reduce or eliminate unwanted ring flutter. This is a phenomenon that can have an adverse effect on engine performance and can potentially lead to high levels of blow-by. (Blow-by is the escape of combustion gases past the ring).
The reduction in pressure in the accumulator groove also has a beneficial reduction in pressure above the second compression ring. Stabilising both compression rings will improve the sealing performance of the rings.