It describes the relationship between strength, effort and capability -- terms with unique but interrelated meanings in exercise. If you are looking for the most no-nonsense summary of the point here, it can be said that high intensity strength training - in its MOST ideal form - is ALL about exercising with adjustable resistance that can be made heavy (or light) enough to allow for a desired (relatively high) level of effort, where the user can choose an appropriately high percentage of their maximal capability, at any time over the course of the exercise.

The MOST efficient, practical, and intelligent strength exercise is one where effort is as close as possible to capability and also allows for the completion of a prescribed number of movements (repetitions), or a portion of those movements.

Understanding this point is the key to strength machine design, despite what you may be told by other so-called strength/fitness experts. You are NOT likely to find an adequate Understanding of that point in most of today’s gyms and fitness centers.

Mindbending question for experienced exercisers in a standard 1 set/10 rep scenario: "If I'm strongest at the beginning of the set, and weakest at the end of the set, shouldn't I be lifting more weight in the beginning, and less weight at the end? ... Shouldn't the weight change with my changing level of strength Through the set?"