Jenny slowly walks toward him, standing behind him. Of course, she is curious as to what he is talking about, as to how this contraption fits into his plans. But she stays focused. For the moment, the Master is concentrating on his device, which means she is an afterthought. If she can just manage to knock him out, then she can run...
But what if the device is closer to completion than Jenny thinks? What if she leaves, runs off to find her dads, and it's too late? Of course, she can always stick him behind the force field, as he did to her, but surely he has a fail safe for that. He's a Time Lord, like her dads. What can contain him, really?
Jenny knows she's running out of time, that she has to make a choice. So she continues to walk up from behind him, slowly and quietly, preparing to knock him out...