Hm.
1)So is it the case that for power techniques, the greater the power, the greater the efficiency, while skill allows you to operate at higher levels of power?
Or is it that learning to channel larger amounts of power into your skills makes you worse at channeling smaller amounts of power into them?
2) Is there anything that can be trained to reduce the cost of these skills, or at least increase the amount of boom for our buck?
3) What do channeling and explosives skills actually do?
1. Yes. Though not entirely static, in general a level-up in a power skill allows you to use 5% more SP, making it about 5% more effective. Overloading a skill above its skill level is a fun way to blow up. (Note this 5% as of this moment is compounding. We'll see how well that works later, and if it needs to be made non-compounding.)
It in no way makes it harder to channel smaller amounts. What does that is max SP, and affinity. Channeling lower percentages of your SP into a skill is harder the more sp you have, for obvious reasons, it's plain out harder to control.
An affinity for the relevant element helps with this though. And as your affinities should raise as quick or quicker than your max SP, that'll probably not be relevant.
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2. The relevant elemental affinity. If the affinity is higher, all skills under it are stronger, with the same cost.
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3. Explosives skill generally makes you better at using explosives. It's more of a general skill, because it allows your character to better estimate blast radius, where to aim, when to aim, and such things.
As a mechanic though, it generally just increases the efficiency of explosive skills.
Channeling allows you to use the element in its raw form, to do things. The primary and most notable bonus is coating your weapon in it, thereby increasing its effectiveness by giving it the element. Much like it would in a game.
If it helps, think of the pine resin from Dark Souls, but with far more applications than simply modifying your right handed weapon.