The terminology Circle of fourths is also explained by Karen Cuneo Ramirez in an unnatural way, BECAUSE if you look at the NORMAL commonly published chart, it goes sharps to the right and flats to the left, intimating forward for sharps and backwards for flats, the much more natural perception of pitch.
The most common chord change is a tonic to the dominant seventh, this being the basic TWO chord sequence, the commonest chord to make it a three chord sequence being the sub dominant.
Also note the relative minors are displayed in-line with the chord they relate to, e.g. C - Am, F - Dm etc.
Be aware that if you use the circle to work out minor keys, the 5 chord is nearly always a 5/7th Dominant seventh and the sub dominant is nearly always a 4m (Minor).
It's actually a series of calculations as to what chord comes next, ONCE the first change has been detected..All based on probabilities.
~It could be displayed as a chart of "either/or" and percentage probabilities..once started in your memory, it just builds and builds, it's essentially basic mathematics.
I'm most definitely NOT criticising the knowledge of Karen Cuneo Ramirez, but her terminology is suspect, and use of it could lead to problems understanding correct music theory.
If you REALLY want definitive theory, then subscribe to the Berklee College music theory classes "on-Line"
Here:-
http://www.berkleemusic.com/music-theory