Ithuriel doesn't really have to move very much to make this more terrible.
Her arm's going to be doing that just fine on its own.
It is a very instant reaction to the first impact of the flames that sees the angel's arm explode outwards, away from that "surgical site".
Like a bloody seam, everything below her elbow splits open down its length, save for a portion of burning mass in the middle that is violently ejected onto the floor.
Veins and arteries uncoil from their containing flesh as a menagerie of flailing tentacles swatting towards Cessie's arm, clearly seeking the source of the hideous fire.
Something is keening, but it's hard to tell what. The bones?
This looks like it probably hurts a lot.