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.
As far as I have read the manga, I think it's not a learned ability, but rather a power to show that by drinking Pip's blood, she became a full-fledged vampire, so her body started to heal and she became more powerful. Also it seems that the tendril acts on instinct rather than her direct will. Hope this helped.
Schrodinger specifically said, in that very scene, that blood freely given is an entirely different matter than just taking it. Alucard has the same tendrils when he releases restraints, and I think it's the souls of people vampires have fed off of.
Since Pip offered his blood, I think it allowed his soul to act independently and aid Seras, as well as giving her a greater power boost than normal. Breaking the illusion that was plaguing everyone when she ascended was noteworthy when it happened.
The potential to do so was always in Seras but she manifested superior vampire abilites due to the fact of drinking blood for the first time. After that she completed her "inner path" to become a full fledged vampire.
