So after Dorian we go to Phrygian which is a more metal sound, and then Lydian, Mixolydian and lastly the Locrian mode all of which it behooves us to know about if not know inside out and use to spice up our musical ideas.
Then we get to the Harmonic Minor part which is something you need to know about and does have practical implications. Then we get to the secondary dominants. This involves borrowing chords from a related key to the one you are in instead of using the intrinsic chord of the main key. Say you are in C.
The dominant chord of every key is the 5th. In C the 5th is G7. We'll use a A7 instead. It gets technical and goes on and on and on. There is a CD that comes with the lesson and this is very nicely done and gives an opportunity to play over the changes if you have been paying attention to what you are supposed to be using with each piece. You can learn an awful lot here but you will need to diligently apply yourself and have a reference to fill in any gaps.
It’s not the years in your life that count. It’s the life in your years. – Abraham Lincoln