non-txv metering devcices are all referred to as 'fixed' because there is no feedback for regulation as is the case with a txv remote sensing bulb. a capillary tube feed is based upon internal diameter and length. a piston, as you pointed out, allows free flow in one direction, so the opposing forces provide some control, but basically is viewed as a fixed orifice. a fixed md system has to be charged by superheat because there is no control over feed into the coil. the presence of superheat (heat added to a gas) indicates all the freon boiled in the coil and is proof that saturation (presence of gas and liquid combined in the coil as air passes over it). interestingly, as ambient temperatures drop, superheat goes up because liquid pressure drops. this results in less feed and therefore less volume and less boiling.
the bypass feature of the piston is fully utilized in a heatpump. when flow is reversed, the piston is bypassed to allow free flow instead of restricting.