When the bottom piston moves in the lower portion of the chamber, it moves alone, as the top piston remains stationary. Liquid is metered in an amount equal to the volume displaced by only the bottom piston. In this mode, volumes and flow rates like those given by a 3 mL to 10mL syringe are provided.

However, when the bottom piston moves up into the upper portion of the chamber it picks up the spring-loaded top piston, which follows it. This meters an amount of liquid equal to the difference in volume displaced by the two pistons. By using pistons with small differences in diameter, DRD gives volume resolution like that of a 50µL to 100µL syringe, with excellent precision and accuracy for samples as small as 0.5µL.