I noticed she was able to do this after drinking Pip Bernadotte's freely-offered blood. when she was about to die from not wanting to drink blood. after she drinks it her left arm is replaced by a mass of chaotic black and orange shadow matter that usually manifests as a whip-like tendril.
When in the world did she learn how to do that. I have watched it over and over again trying to figure out when she knows she is able to have that happen.