In a KGS game I played tonight, by opponent played a very strange-looking response to my 3-3 invasion, which I'd not come across before (the game as a whole was fairly stupid and not very interesting, but this part of it made me want to ask about it here). I'm talking about the move 4 below. (Both my approach stone and his 6-3 stone were part of living groups, btw, if it makes any difference to the analysis.)
I wasn't sure exactly how to handle this. The sequence below is what happened in the game, leading to a ko. It's not clear to me how either of us could have done better by playing different moves at any stage (all of my moves in particular felt pretty much forced) - but then I'm not always that good at reading life and death problems so it's entirely possible that I've missed something:
So, my questions are:
1) Is this really the best play by both sides? If not, what other options are available?
2) If it really does lead to a ko, how come I haven't heard of this move before? Provided that the invasion (as here) happens late enough in the game that the thickness black gets from the usual joseki would be of limited use, it seems obviously superior for black to play this way than to let white live unconditionally.
Thanks in advance for any enlightenment
(For those interested, in the game I "won" the ko and lived when black responded incorrectly to a local ko threat. But I already had a winning position in the game before I even invaded, so it was probably moot anyway.)
:
. Therefore, white should simply play 
without