What does it mean "going Harder" ? (in a mix :)))
I tried to figure it out to make it clear how does a mix look like, and I wanted to see it as a graph.
Based on the BPM and Key it is possible to draw your mix on a paper. This way you can see if your mix is
continuous or not, and you can define the "Hard" concept too.

You can place each track of your mix on this plane and you can see how your mix progresses.

For more info: The two dimensions of mixing