Constantly varied is a part of the recipe that is often confused.  Constantly varied is not "muscle confusion"! It has nothing to do with "avoiding plateaus".  Maybe the easiest way to get a good grasp on this is to thing about the opposite.  Pattern overload is when you do too much of the same movement and it creates an injury or soreness.  Think tennis elbow which is really just an inflammation of the connective tissue in the elbow, or IT Band syndrome.  These are examples of predictable patterns of injures based on repeating the same movements over and over, and ALL sports/activities have them.

The best way to avoid these overuse injuries, these pattern overloads is to engage in as many different patterns as possible, as often as possible. 

Another thought that is helpful here is the old training acronym S.A.I.D (Specific Adaptation to Imposed Demand).  Your body will adapt quite predictably to the stimulus you give it.  Consider a marathon runner, s/he may have great conditioning but likely is lacking in strength.  The opposite end of the spectrum is a powerlifter.  S/he most likely has great strength and poor endurance.  That is because each is training with a specific goal or outcome in mind.  Both are fine.  But both will also lead to pattern overload more likely than not.  

What we are doing with constant variation is developing the positive adaptation of each while minimizing the negative adaptation.  By programing a steady dose of bodyweight/gymnastic movements and kettlebells and Olympic lifts and traditional cardiovascular training all done with varied rep ranges and intensities you build ALL of them.  

So the ultimate goal of constant variation is you build a strong, resilient (nearly bulletproof) body, capable of doing anything, at any time, for any reason.