Just as important is that the joints are used in the correct order like following the links on a chain. For example, in the most basic terms for a beginner, a mae-geri keage starts with the toe joints of the kicking foot; then the ankle of the kicking foot and the heel of the support foot; from there; from this point, the knee joint and hips (and the natural contraction of frontal abdominal muscles, one of the many forms of shime, which I won't go into here); and finally, the knee joint to return/recoil the kick. Of course, and once again, this is the most basic type of example and does not include many utterly integral aspects such as shisei/setting your hips/spine/posture; chushin; the aforementioned and `seemingly paradoxical' aspect of shime, and so forth.
Taken as a whole, 'power flow' essentially allows one 'to move primarily by their technique as opposed to forcing their actions'; accordingly, it therefore highlights such old adages as "...technique is more important than strength" and "less is more". Returning to the example of mae-geri, we can often see lower grades kicking hard with brute muscular force, whilst higher ranks (who are deserved of their grades) are 'relaxed and whip-like'. The lower grades are in fact fighting themselves when they use brute power, as they are using both sides of their muscles (e.g. - simultaneously contracting, say, the quadriceps and hamstrings) and, thus, limiting the velocity of the attacking limb (what is often termed as `self-resistance' here in Japan). What's more, they are destabilising their entire body by exerting unnatural force and, in particular, exerting unhealthy pressure on their bones, ligaments and tendons. Conversely, the higher grade using power flow, is faster, their torso is more stable, their output/impact potential in general is much greater, and they are less likely to acquire injuries. Indeed, there are even more advantages... Power flow, as mentioned before, allows one to use the joints more freely, more aligned, more smoothly and in the correct order. What's more, it frees the mind, as when one forces techniques the tendency is to become `mentally locked' their own action as opposed to having optimal zanshin.
Actually, there are many other advantages of power flow, such as the ability to execute unpredictable henkawaza (continuous/combination techniques) and the capacity to develop top level deai-waza (meeting techniques), but I will leave these for you to discover via training. To help prove my point, think of the women's kata at the JKA (Japan Karate Association) All Japan Championships... How and why can those lightly framed Japanese girls move so sharply/powerfully and have such technical precision? Even now in 2015, no one comes technically close... Why is that so? Is it because they are small? Is it because they are Japanese? Is it the Japanese physique? No! The answer is that they are fully expressing power flow in their techniques, which comes from correct training of Karate-Do technique. Another great example are the children's kata divisions. The reason they are often so sharp is quite interesting... It is because their muscles are underdeveloped!! Think about that for a moment, and self-apply: if you do you will really improve your karate... I'd like to stress here that I am 'by no means saying muscles are bad'. On the contrary, the stronger your fast twitch muscles are, the better you will be. My point here is that, "...stronger muscles are stronger at holding you back if you use them incorrectly"; that is, without using power flow.
Away from kata, a basic experiment is to engage in jiyu-kumite with a senior grade, who is a skilled kumite exponent (I'm saying that for mutual safety), and try to punch, kick or strike them with all your might/muscle strength. If they are indeed skilled, you will stand no chance against them. Contrastingly, then change into a complete state of physical relaxation: it will be immediately evident that this is infinitely superior. This brings a major question to rise... Why are so many karateka relaxed in kumite yet stiff in kihon and kata? Probably, they answer is a lack of physical understanding and/or awareness of power flow. Irrespective of the reason, it is critical to know that power flow is not a theoretical point. It cannot merely be cerebral. Likewise, the trinity of the `The Three K's (Kihon, Kata and Kumite) cannot only be something people give lip service to. One must practice techniques over-and-over again, year after year, to make it a physical reality that permeates
through every aspect of one's karate. Consequently, the essentiality of the foundational techniques--and high repetitions of them--is once again vividly illustrated.
In sum, real power in Karate-Do comes from correct technique, which is primarily from the proper positioning, and moving/synchronisation of the joints, which can only derive from power flow. Yes, the muscles are what move the joints; however, power flow can only be achieved when unnecessary muscle tension is eradicated. The wonderful thing with power flow is that it allows the muscles to naturally/autonomously do what they are meant to do: in the best way possible. To further illustrate this, imagine the professional boxer, tennis player, and other such high level athletes. You never see them consciously tensing up. Yes, you can see the muscles contract when the boxer connects his punches. And yes, you can see the calf muscles of a tennis player tense when they stretch their stance (when launching across the court to return an incoming serve). But neither the boxer nor the tennis player are consciously tensing in these actions, the muscles are simply tensing based on the action being performed. I'll repeat that one more time: tension is the result of action only, not a conscious point of/within the action.
Here's a simple test (to establish your power flow and make some serious corrections/improvements): Make a kizami-zuki with your fingers extended, like a nukite but loose. Whip it out and back as snappily as possible. Now, do the same thing with a normal kizami-zuki; that is, from a proper kamae and with a properly formed seiken. If the speed of your normal kizami-zuki is not close to identical (to the first exercise) you are definitely too stiff. With a little practice, you can achieve identical speed. This is merely step one. The next point is to be able to do the same with age-uke, mae-enpi, yoko-geri keage, yoko-geri kekomi, shuto yokomawashi uchi etc...
On the whole, irrespective of how one labels it, 'power flow' is the most important aspect of using natural energy in Karate-Do. It allows the full expression of the joints/intersections of the human body via junansei (softness) and, just as importantly-if not more, it permits the muscles to work in an optimal manner to achieve this objective.