Abstract: An improved mechanism is provided for selectively altering the positioning of an inwardly depressable, outwardly biased, shiftable structure, such as a splined shaft, and for releasably holding such structure in either of an inwardly depressed or an outwardly shifted position thereof. The mechanism is especially adapted for use with powered wheel, hub assemblies, such as commonly employed in combines or other normally self-propelled agricultural implements or the like, in which provision is to be made for disengaging the hub from an associated power shaft to permit towing of the implement at higher speeds than normal for field running thereof.