How do you use painting medium?

[Jeff Fuchs]

When I was in school, I was taught to mix painting medium with the paint on the palette. I have, on occasion, read that medium should be applied to the canvas, then painted over. Yesterday I found a painting demonstration that urged painters to return to the lost technique of applying the medium to the dry canvas (or dry underpainting), and then painting wet on wet into it.

Do any of you use this technique? It seems logical to me. I am not painting yet, but I want to know these things in advance.

Here's the demo:

http://studioproducts.com/demo/demo.html