What does Irisalis's Dance Power do?

Stacks only come from clone hits, so a volley that misses adds nothing toward the nine.

Every Bloom Cluster volley that connects is worth one Dance Power. Once you reach nine, the discount applies to her next skill only. Spend it on the skill you most want to cast cheaply rather than whatever comes up next. That matters more for Irisalis than for most, because she has high EP consumption. Dance Power is the part of her kit that gives some of that energy back.

Anything that keeps her clones landing hits keeps the cycle running. Her exclusive signature held item, the Floral Rainbow Feather, is built around those same clones. Its core tier 2 effect gives them a damage and crit buff that includes 30% damage amplification. So the hits that build your stacks also hit harder.

Not settled: It is not settled whether nine stacks halve the next skill's energy cost or take a flat 50 EP off it. On any given skill those two can give different discounts.