When your aura ticks, or when you cast a healing spell, it runs a calculation. (Base healing plus caster level as applicable) x (100+spellpower/100) then there's a chance of a spell crit (equal to your spell critical chance); if you get a spell crit, you multiply the whole thing by (200+spell crit damage)/100.
Using Holy Presence as an example, if you have CL 25, 600 spellpower, 40% spell crit, and 30% spell crit damage: (1d4+1) x 25 (CL) x (600+100)/100.
1d4+1 averages 3.5, x 25 = 87.5 x 7 = 612 average proc. However, 40% of the time it'll crit, and those times it'll be 200 (base crits are 2x) +30 (your bonus spell crit damage)/100 (to remove percentages = (200+30)/100 = 2.3x the base proc = 1409 average crit proc.
So taking ESP: Positive gives +20 spellpower (which would make that 620 spellpower instead of 600, which is a 2.85% increase in heal size) and +10% Spell Critical Damage (which would mean you have +40% bonus, or 2.4x instead of 2.3x = 4.35% larger crits as well). In the above example without ESP: P you would average 931/tic (given the crit rate), and if you took ESP: P you'd average 983/tic, a 5.56% increase on average.
The more crit chance you have the more effective crit damage multiplier is; a melee build using EA mantle while leveling that only has base 5% spell crit chance isn't going to care much at all, but a dedicated healbot with 70% positive spell crit chance will be getting a lot more value from it.
Does that help clear things up a bit? TL;DR: spell crit damage makes your spell crits (which are base 2x) bigger whenever they happen.
(Edit: how do I get markdown mode here so I can stop having emojis show up)
