The use of fuses allowed the device to be programmed electrically some time after it was manufactured and sealed into its packaging.
Earlier fuses had to be blown using a laser at the time memory was manufactured.
The first polyfuses consisted of a polysilicon line, which was programmed by applying a high (10V-15V) voltage across the device.
The resultant current physically alters the device and increases its electrical resistance.
Modern polyfuses consist of a silicided polysilicon line, which is also programmed by applying a voltage across the device.