The Cosine Mix texture is used for mixing two textures together according to a cosine wave. It is very similar to the Mix texture, the difference between the Cosine mix texture and the Mix texture is more apparent when the amount slider is shifted towards 0 or 1.
how to use
Open the Node editor and prepare the setup as you see in the picture below. In this setup, two separate procedural textures are defined (marble and checks) and both are linked to the "Cosine Mix" node. An image texture was used for the amount slot. You can define any procedural, alpha or image texture for the amount channel. If you define Alpha image, only alpha of your image will be mixed. This applies to all mix processes in Octane. But you can also play with the slider if you like. It is a simple setup and you can use it many scenarios like this one.