The present invention is a device and method for controlling longitudinal movement of a tube relative to a shaft slidably disposed within the tube, especially in the catheterization of a patient. An operative segment on the shaft cooperates with an ancillary tool to create a coupling force field between the shaft and the tool. The tube can then be moved over the shaft while the coupling force field operates through the tube to restrict the movement of the shaft. In a preferred embodiment, the shaft is a guide wire and the tube is a catheter with a lumen for slidably receiving the guide wire, while the coupling force field is created magnetically. In one embodiment, the operative segment is borne on a short guide wire extension which is selectively securable to a standard guide wire.