Sort of gone and got his hand cut off. It's the most casual attitude towards dismemberment Cosima's ever heard, and both her eyebrows are up now. It's definitely true that she has little idea of clumsy swordfighting and losing hands, but she's drawing connections anyway, mind flitting from possibility to possibility as he speaks.

She huffs a wry little laugh through her nose at the embarrassment in his tone about the hand in a jar, and shakes her head a little. 'Wow, and I thought my life was complicated. Ok, so, lemme get this straight; this Doctor gets shot, which triggers the regenerative process, but-- the hand introduces an extra variable? Or--'

She can't really imagine how a severed hand in a jar would interact with an internal biological process in such a way as to produce a clone, even if the process is one which releases a lot of energy. Unless, and she exclaims softly to herself, 'Or, oh, oh, if the hand came into contact with the Doctor, then would all that volatile energy kickstart cellular regeneration in the hand as well? That, I mean, that's crazy, but, well, this whole thing kinda is already.'