No, you're not missing anything..... I just did not know what else to call it besides 'displacement rate'. It's the change in volume in the cylinder as the piston moves down..... in essence, the 'flow rate' if there were were no restrictions in the path of the flow in or out. The peak is the maximal rate of volume change, and is simply the rate of change in the piston position... better known as piston speed!
