Ben has taken a seat in the maintenance passageway, just by the hangar, his presence now invisible and his mind free to focus, to listen to the Force as it flows through and around him and the two Sith.

When things start happening that he'd have an emotional reaction to, were he not a Jedi, he lets that emotion dissipate and focuses instead on what he can sense, observe.

He'd forgotten Ibani's particular skill, her fine point accuracy that even Vader, with his unbridled power and Yoda, with his cenuturies of study, couldn't get close to, and he chooses to watch that, to study her technique, rather than to react to her use of it.