It has a certain physical effect and does not contain any toxic chemicals.

By its strong spreading action, It allows to go around vegetation and other obstacles on the water surface and is highly resistant to wind or wave action. 

It is also not effectted by rain falling on the surface.

It spreads on the surface of still water and even forms a very thin film layer in large water bodies. 

Bacause silicones have a very low surface tension, the film prevents the pupae and larvae from attaching themselves to the surface as they attempt to breath, thus causing them to suffocate.