I have adopted this Boomer axiom, and find it answers a lot of questions for me regarding kick timing. Whether it is the fundamental he means, or the second harmonic, both have relevance when you need to conjure a mental image of what you are trying to achieve.
The search for the perfect two-beat kick is now a tangible exercise in tuning rather than kicking harder or throwing the arm forward with more force.
To exert the force you must first master the timing and then 'for one moment must every instrument exert its fullest force'. Not sure Elgar was a swimmer, but it seems apt! :-)