The animation below shows a cross section of a piston ring installed in the groove. This view shows the ring in its sealing position. Pressure differential is established between the high pressure side of the groove at the top part of the figure, and the low pressure side at the bottom part of the figure.

Primary Sealing: In the case of an expanding ring, primary sealing is established at the ring outside diameter (OD) surface and the cylinder bore. For contracting rings, the primary sealing is formed by the inside diameter (ID) making contact with the shaft/rod. This contact is maintained by the inherent tension built into the ring until pressure differential is sustained across the ring. Pressure differential keeps the ring "seated", forming an effective sealing action.

Secondary Sealing: Is established at the ring side face in contact with the groove wall. The desired clearance between the ring and its groove forms a flow channel that carries fluid to the back of the ring to establish pressure differential. Once pressure differential is attained, the unbalanced reacting forces at the ring sides and primary sealing faces cause the ring to form an effective sealing action.  

The same factors that contribute to seat the ring also cause drag forces. Axial Drag Forces are a result of the high pressure fluid acting on the I.D. surface of the ring and the coefficient of friction between mating surfaces. In addition to this force, one must consider the drag forces due to the ring's inherent tension. On very high pressure applications, these drag forces become almost negligible by comparison. For low pressure applications, rings must be carefully designed with drag forces in consideration. Another type of drag force is caused by friction between the ring side face and the groove's side surface. If this Side Drag Force is too large, the ring will not achieve primary sealing contact. Again, for low pressure applications, this drag force must be carefully balanced while designing the ring.