The Muscle Confusion Principle is codified in the Weider Principles, which are a list of guidelines that bodybuilders in decades past developed. These "guidelines" or "principles" are not based on scientific studies. They are the result of less educated men experimenting with trial and error and often contain an element of what's been called "bodybuilder logic" - it looks like logic, but isn't. These ideas are still popularized through Weider publications to this day, especially Muscle & Fitness. Some of them can work, but only if you understand the scientific basis as to possibly why, and even then its still a sort of "maybe."
Muscle Confusion is a kind of wording that extends from a belief (or a model, but they didn't call it that) that Arnold Schwarzenegger and other BB's had that muscles adapted to stimulus - how you lifted - and eventually became resilient to the stimulus. So in their way of thinking they concluded that one had to "surprise" or "confuse" the muscle with something, anything different. If, and then why, it works is difficult to say. The body is complex and hypertrophic signaling is likewise complicated. Its entirely possible that by changing lifts, rep ranges, tempos, order, eccentric/concentric/isometric that a bodybuilder could see a positive result. Controlling that result and predicting how it can be made to happen is anybody's guess. One explanation is that by constantly varying their workout the old school bodybuilders randomly struck upon known growth principles, some of which are in HST.
The idea hasn't completely gone away. Westside Barbell routines call for changing exercises in the hypertrophy aspect of the training every few weeks. Charles Poliquin, an Olympic level strength coach and entrepreneur, thinks that after around 6 workouts a lift begins to get stale. This is especially true for people of intermediate training age and even moreso for people of advanced training age. Charles in fact has stated that elite level strength athletes must change their lifts every workout to make progress. Ronnie Coleman, 8 time (or is it 9?) Mr. Olympia champion will alternate the lifts every workout day. He states, for instance, that if he does back squats on Monday he will do leg presses or front squats on Thursday.
For beginner's: Don't stress yourself over it. You ought to be trying new lifts every so often anyway just to learn them and improve your athletic skill. The "Variety" or "Confusion" principle is mostly used by advanced groups who as a rule have more difficulty improving than novices - hardcore powerlifters in the case of Westside, Olympic althletes in the case of Poliquin, and professional bodybuilders in the case of Coleman.
