John is sharp enough that he can tell Ianto is being evasive, mostly because it's the kind of evasive wording he uses himself. Like recognises like.

"If it's all sciencey, don't worry about the details," he says. An out, of sorts, if Ianto needs it. "I know not everyone is as brilliant at diagram drawing as I am."

A topic change as well, if he wants. John is happy to redirect onto himself as a joke if need be, it doesn't bother him. He'd be a hypocrite if he forced Ianto into discussing things he didn't want to after dodging so many things himself.