She basically hijacks control of their magic.
"I can see the energy that's propelling you. There's just a little remaining, enough for maybe two more trips. It's going to need some time to build up momentum, but that's the thing about magic. Once you see it, you begin to understand it. And once you understand it, you can affect it." His magic surged and Darquesse gasped, felt her arm suddenly throb, and then the world flickered, flickered, and he reached out for her and they were standing in darkness, in ruins.
That example is someone else taking control of the magic in her to teleport them both to another universe. He did it, but she absorbed his knowledge and used it to gain his powers.
She looked closer, deeper, saw how Necromancy inhabited it. The armour was a living thing, though not a living thing with any degree of sentience. Magic lived in it, and magic was alive, and so the very nature of the armour was transformed as a result. She saw it and understood it, and once she was ready, she focused her mind and called to it. Vile inclined his head, and stopped tapping his leg. She called again, more forcefully this time, and the armour reacted, straining against Vile's command. Black droplets of shadow splashed to the ground and Vile struggled, the armour turning solid again, solid and sharp. Darquesse ducked a swipe at her throat. She moved in but he was fast, grabbing her and twisting and smashing her head through the wall. He pulled her on to his hip and flipped her. The world tilted and the floor spun into her face. She started to laugh as Vile knelt on her back. This was fun. She threw him back with a simple pulse of energy and she called to his armour and it flowed to her hand, collecting into a spinning spinning ball of shadow and substance.
That is Darquesse overriding Vile's control of his own necromancy. This is an extremely good feat, as Vile has extremely sophisticated control over his own powers, and is one of the most powerful characters in the setting.
Another roar from Star as she staggered, her magic boiling within her. Darquesse could have killed her from where she lay on the ground. She could have burst her like a balloon, or taken her head off with a stream of energy. She could have got up and grabbed her and torn her heart from her chest. She didn't do any of that. What she did do was to reach out with her magic and smother Star's, keeping it from boiling over. She saw the confusion in the renegade girl's face, as something that should have happened didn't happen.
Third example is the most "power nully" of them all, where she just stops a girl from using her power by "smothering" it with her own. This girl had her magic boosted by the Accelerator by the way, which allowed Lord Vile to go from getting his magic hijacked and curbstomped, to almost killing Darquesse, so it is quite the boost.