This is because visco style fuse, using a fast burning black powder, generates thrust as it burns and in small amounts can therefore propel itself.
Cut into small sections, visco style fuse can make “flying fish”. Flying fish produce an effect similar to a tiny rocket but they are not stabilised by fins or rotary motion. The result is a sparking, glowing dart which flies off in any direction it wishes (as seen below) and can be used in mines, shells and rockets.
Fuse spinners also take advantage of the thrust generated by visco style fuse. Simply bend the fuse into a curve, or producing the visco in this manner, will form a spinner. This is similar to the flying fish effect but the direction of the thrust is projected at an angle to the rest of the fuse body. This spins the fuse and prevents it flying off in any given direction. This produces a simple spinning effect showering sparks in all directions and can be used in the air and as a ground effect, as shown in the video below. The video also shows a simple flash powder report added to one of the spinners for an extra effect.