In her own mind, the female Jedi Knight heard the message from her mysterious ally through the Force: If he survives this, we can hold him down with the Force. If we want to save Varik, one of us may have to journey into his mind.
In the split second after she heard Tulak Hord's threat, she responded: That's a fantastic idea, but the big question is: Will WE survive this? If Varik--er, Tulak Hord--wins, he's going to kill us all if we're not already dead... Nevertheless, Per'dra still held to her previous plan: fight or perish. If there was one thing she knew about Sith Lords, it was that they loved to prove how weak and pathetic the Jedi were. She wouldn't let Tulak Hord have that satisfaction easily! All of a sudden, she had an idea: Even though she'd been previously trained as a Consular, she'd learned a new technique since then and could cloak herself in the Force and become invisible. Thus, in a flash, she did so.