Functionality is simple (and rather limited). You can choose whatever blend mode you want out of the default ones. The idea is we are are creating a rectangle the same size as the image and we can set whatever color and blend mode we want for that rectangle. That way, we are able to see the effect it has on the photograph.
The main feature is the custom blend mode. We can write our own formulas and see what a new blend mode would look like.
Some considerations: the language or syntax we are using to write this is GLSL. Obviously, I don't expect anyone using this to know what it is, but there might be a chance that you know. This language has a strict set of rules that can (and will) break when you try to input a formula of your own. The program crashes and stalls, so just reload the wepage.