Power Strike: Attempting a hit with a melee weapon, either close or extended range (who knows if it matters if you hit or miss?)
Powercast: Using any spell is good enough, though I think it depends on spell level and power level.
Iron Will: Resisting a spell is good enough? If affected, how quickly you recover?
Iron Skin: The worst of the bunch. You actually have to be hit for it to register.
Reflextion: Apparently, dodging an attack is good enough. Who knows how it works if using a shield.
Snake Speed: Just taking an action is good enough (even defend counts as an action).
Eagle Eye: Attempting a hit with a ranged weapon, whether that is a gun, bow/crossbow, or sling.

Powercast increases the duration of non damaging spells.
Powercast 100 results in 25% higher raw dmg compared to powercast 1, i.e. 25% above the theoretical max dmg.
The test was performed with new spells that cost 360SPP/PL,
so they ignore the targets resistance of 255 in the related realm,
unless the targets Exp Lvl is far beyond(~ 50 lvls) the casters Lvl,
This does not take into account that powercast also helps to pierce resistance
if the spells mana cost is within range of the vanilla spells.


Snakespeed:
Decent Snakespeed (30 to 40) gives characters an additional attack with 2 swings
if the weapon they use has low weight. (3 lbs is probably already too high)
That bonus attack is performed before the actual initiative sequence.
If all party members are that fast, the game also allows to always move before enemies act at all.
E.g. a PL7 supemanned snakespeed50 ninja has shuriken in both hands,
a click on sword/shield button(personal inventory page)
shows 3 attacks with 3 swings for each hand, but in game you can observe a 7th attack,
and the 1st attack of all has never more than 2 swings.

Actually, I seem to remember its growth not being affected by spell level and power level. Does this, by any chance, include Death Cloud? (Unlike the other cloud spells, Death Cloud doesn't deal damage.)

Also might be worth testing with Ring of Fire and other cloud spells. Why did you test against 1 rather than 0?

Worth noting that having the expert skill, even if it's at 0, provides a bonus that you don't get if you don't have the skill, or if you somehow have points in the skill without qualifying for it. (That last point can happen if you equip something that boosts the skill, or if disease lowers the attribute from 100 to something lower.)

(This means that items that boost expert skills, like Caliban's Cuirass nd Snakeskin Boots, are less useful for characters who haven't unlocked the expert skill in question.)

The test was about how much SPP/PL is required to pierce a certain amount of resistance,
not about the powercast related increase of duration that can already be observed with vanilla spells,
so i did not bother to check that out. If forced to bet i would say yes.

I tested vs 1 powercast as i was too lazy to edit, also not sure if the 0 is applied as a multiplier in the algo,
although if forced to bet again i think its not, i.e. powercast at 0 is probably better than no powercast at all.

btw death cloud: The spell has proven to kill monsters with realm resistance at the maximum of 255,
while no other vanilla spell caused dmg or a side effect on the targets, unless target lvl is far above casters lvl

Just unlocking an expert skill, it starts at 0 skill. There is an immediate benefit. For example, Powerstrike skill of 0 gives +1 to hit for melee attacks. Every 20 points after that is an additional +1 to hit. This means that 80 Powerstrike skill is the same as 99 skill, giving +5 to hit. That's why getting to 100 skill matters, as hard as that may be.
Edit: Also, regarding Powercast. Using lower level spells like Guardian Angel, Stamina, Rest All, Chameleon, Fireball, etc. are doing just fine at raising it. I don't believe that I have to use the more costly, higher level spells to raise PC then.