Disclosed is an apparatus for a rocker. An implantable medical device may be configured to determine an anode capture threshold and a cathode capture threshold for a first anode and cathode pair of electrodes, switch a polarity of the first anode and cathode pair of electrodes, and determine an anode capture threshold and a cathode capture threshold for the first anode and cathode pair of electrodes having the switched polarity. Accordingly, extremely high speed conveyance of the articles or mail pieces, within the range of, for example, two hundred inches per second can in fact be achieved. Only a single trip of the flow tube exercising tool is necessary to accomplish multiple upward and downward movements of the flow tube.