It turns out that fractional amount of a single second is not actually plenty of time for a human body to react in, which is the kind of thing that probably would have been obvious to anyone except Dirk, and the window during which that lapse was excusable passed a couple months ago.

As it is, he's extremely lucky that Jane's act of self-defence doesn't clobber him right in the dick. The stray Kartana between him and her zips downward, perhaps swept out of the way by simple air displacement, and it's a matter of a few inches that she still socks him in comically vulnerable junction between his upper thigh and his torso, right below the hip where no amount of muscle is really going to stop it from hurting a whole fucking lot.

Jane, Dirk, chair, and somewhere around a dozen Kartana go in all directions. The chair goes skittering, spinning off to the side, Dirk's shades and hat go flying, he tucks and rolls and keeps rolling, catching his hat on his head but not the glasses, at least not before they hit the floor--

You get the picture.