like absorption of spell energy right?
afaik you get energy per enemy attack.. if it scales off enemy damage or energy cost of spells cast by monsters, I'm not sure.. but i think the 2nd one (energy cost)
it feels weird though because you benefit most from it if you're a melee.. but melee toons don't need much energy.. energy pots are usually enough.. energy gain is useful for a caster but if you're a caster, you should try not to get hit or not to get hit much.. so unless you're a weird hybrid caster melee, energy absorption seems an odd source of energy gain (i think)
I'd much rather prefer energy leech, reduced energy cost, or %energy regen