Taken from Popular Hotrodder:
An increase in rocker-arm ratio nets more than additional lift. It will also change the cam's duration characteristics. Because the increased ratio effectively speeds up valve movement, that means the valve will reach any opening height sooner than it would with a lower ratio rocker arm. Higher ratios open the valves quicker and close the valves a little later. Since the increase is symmetrical on either side of the cam lobe, centerline a higher ratio will lengthen the overall valve timing making your cam act bigger. The higher ratio causes valve timing to increase proportionally as the valve opens further