Can anyone explain the M effect of HBC or HRC fuses?

M effect stands for 'Metcalf' effect after the person who discovered it. It involves the use of a small amount of low-melting point alloy (often solder) fixed to the fuse element (wire or strip). Under low overload conditions the fuse element heats up causing the M-effect alloy to diffuse into the metal of the element, increasing its resistance and hence shortening the time to blow compared to the same element without M-effect. Under high overload conditions the fuse element heats up so quickly that there is no time for diffusion to occur. Surprisingly, fuse designers use this effect as a way of creating a fuse with time-delaycharacteristic, because it allows them to use a thicker wire (or strip) than they would otherwise be able to for a given current rating. It is this thicker material that allows a time-delay fuse to withstand short-duration high- current surges.