Thanks to
@Bommelom for helping me refine these thoughts from above.
How I think Ki Enhancement Works
An Analytical Speculation
There is in every body a ki network, through which ki circulates. Ki is generated by the body, and also used by the body for enhancement. Generally it takes a while for the body to consume the ki generated, long enough for the body to replace it, but the rigors of a fight use up ki faster than it can be generated, which causes exhaustion. Several times fighters have commented about having "used up all their Ki" or "used up all their energy".
For the wider Galaxy, Battle Power or Power Level (called BPL from here out) is then a measure of Ki Flux (possibly flow) in the body. More ki, or faster flow, or better distributed flow throughout the body can all make a person more powerful.
Of course, while the wider galaxy knows how to do ki blasts, they don't know how to optimize any of this beyond whatever the genetic and environmental lottery sees fit to bestow upon any given individual.
Not so for martial artists of Earth and Otherworld. I shall attempt to use this model of a Ki Circulatory Network to explain the various shenaniganry Dragon Ball Martial Artists get up to.
Suppressing one's BPL is obviously a matter of throttling back how much of your ki is flowing, and raising your BPL is increasing how much of your ki is flowing (naturally up to a given maximum). This would be Ki Control in the quest
Ki blasts are also obvious, simply having the ki flow out of the body in a destructive way. Blasts are discrete cut off quantities while beams are more like a water hose spraying water.
Muton Roshi's training with the turtle shell, or the gravity room training is somewhat different and requires more explanation. Like any system designed by evolution, the Ki Circulatory Network is horribly unoptimized, and Mutaito figured out a method by which one can adjust the automatic and subconscious management of ki flow to more optimal arrangements. The Turtle Shells and weighted clothing and the gravity room are, presumably, all methods of assisting this process.
So what about that incident with Ginyu in Goku's body not being able to get his BPL nearly as high? I posit that the methods of the Mutaito derived schools include some sort of concious control over how your ki circulatory network distributes your ki. Dancing is different from brick laying is different from feats of pure strength is different from digesting is different from fighting is different from gymnastics. Which places in your body have priority on getting ki will influence which of these things you are better at. Thus part of Mutatio derived Martial Arts schools is influencing consciously how the ki network distributes ki throughout the body. This also explains how charged Ki attacks can have a higher effective power level.
Kaio-Ken is increasing the rate of flow through the network. As mentioned in the show, this is extremely delicate and while it gives enormous boosts it comes with equally enormous stresses on the body. The Ki Circulatory Network is designed for a certain rate of flow, and increasing it is something that it has trouble handling. It is possible to mitigate these stresses with good enough Ki Control and Distribution control, which is how Goku is able to master x2 and then up to x10 without any of the usual danger. "Kaio-Ken Times X" simply means your ki is flowing X times faster than normal.
Bulking is increasing the cross sectional area of the Ki Conduits that form the network. Because Ki is bullshit that doesn't follow conventional physics, this doesn't decrease the flow speed and thus has no relationship to the Kaio-Ken. Bulking
would be related to the Kaio-Ken, but having that sort of relationship messes with things when Zenkai or similar training comes around, which clearly does not match what happens in the show.
Zenkai is the Saiyan body deciding to produce more ki than it did previously after surviving a beatdown, because clearly it wasn't producing enough before. I presume that the Turtle Shell and Weighted Clothing and Gravity Room also help train the body to produce more Ki, thus when the need to reinforce the body against the weight disappears some amount of ki is freed to increase the amount flowing through the body being used for enhancement in other ways. Because Ki is bullshit this increase in the raw quantity of Ki doesn't cause any sort of pressure problems for the fighter's Ki Network like Kaio-Ken does.
Super Saiyan would depend on which system you're using. If it's a multiplier, then it's probably a well of additional power. We are using Bringer of Death's system, so it's probably a second Ki Circulatory Network attached to a new well of power. The Base Cap increases are a result of how these networks aren't completely independent since it's the same body, and FPSSJ is achieving the same kind of control over the second network as you have over your base network.
I have not considered deeply how Higher Levels of Super Saiyan would work in this model. SSJ2 would presumably be yet another network, and SSJ3 accessing even further redundancy in the Saiyan Ki Networks, at least for Bringer of Death Based Systems. For multiplicative systems it would just be even bigger wells of power, or figuring out how to draw more power from the SSJ well per unit time.
SSJ4 isn't really properly a continuation of the SSJ line, so it's probably more related to Oozaru than SSJ proper. Maybe it's a combining of the Oozaru/Base/SSJ networks to a single unified whole? I dunno.
Edit:
I am clearly wrong about Kaio-Ken and Bulking being unrelated. Ki is demonstrably capable of generating thrust, Kamehamehas are used multiple times for that purpose and Bukujutsu is fucking flying. Thrust means Newton's Second Law or something like it applies, which means Bernoulli has something to say, which means Kaio-Ken and Bulking are related.
Bulking therefore is increasing cross sectional area while increasing flow speed back up to normal, and Goku bulked up when using Kaio-Ken x3 against Vegeta to help mitigate the stresses of the technique.
Ki's magical bullshitness comes in when a greater density of Ki doesn't increase the pressure or velocity or whatever of the ki conduits.
Edit2: Newton's Third Law applies, not necessarily his second. But I still feel Bommelom's conjecture of Bulking and Kaio-Ken being related is an elegant way of tying disparate things together.
I imagine they'll be on the patrol for the next while.
