This is a summarising report on a recently discovered melt flow singularity during capillary extrusion of polyethylene including the description of the effect, the principal parameters affecting it, the functional relations which arise and the explanation which emerges together with some significant still open-ended issues. The singularity itself is a sharply delineated minimum in flow resistance within a narrow temperature window observed in the course of extrusion of high molecular weight polyethylene. This ＇window＇ effect, leading to smooth extrudates under conditions where otherwise distortions would arise, can be tracked down to the formation of a ＇low viscosity＇ phase conclusively identified with the ＇mobile＇ hexagonal phase, itself arising through flow-induced alignment of presumably adsorbed molecules, this alignment being critical in strain rate and molecular weight in a quantitatively definable fashion. This ＇window＇ effect thus links up with current theoretical trends in melt flow focusing on events along the boundaries of flow channels, while on the other hand it opens up new practical possibilities. From the applied science point of view it offers a new handle to the more general understanding and minimisation of extrudate distortions beyond the confines of the window effect. Amongst the open-ended issues are two power laws, specifically precise - 4.0 and - 1.5 power laws in molecular weight defining the onset strain rates for the ＇window＇ effect and for gross shape distortions respectively, both of direct practical consequence yet with pointers to some still unexplained basic features of polymer melt flow.