Total internal reflection occurs when light within high index of refraction medium is incident upon an interface with a medium of lower index of refraction at a low enough slope that the light cannot refract into the other medium and instead is entirely reflected within the high index of refraction medium. This plastic spirally thing demonstrates total internal reflection as light entering in one end seems to twist with the tube and (mostly) only exits at the far end.
Shining laser light in one end...
...it comes out the far end.
