Black just played the marked stone. If White answers with 1 then the sequence to 6 is forced. Now White has two follow-ups.
If White plays 1 to make an eye on the upper side then Black can play 2 and the following moves. Of course White can play the ko at a but in the end it is a ko anyway.
Or White can play something like 1 to make a sure eye. Black 2 is good and again White cannot make eye without ko.
As White is ahead I preferred to play the simplest sequence, even losing some points (very few, really).
Now, explaining this, I realise that White can live in double ko by playing 1. White has two kos, at a and b, and she can live by winning one of them.
Lucky To be frank I didn't even consider this move. A beginner move may be a pro move, they play the same :-))
I do not imply that dnerra is a beginner (or a pro by the way), just that Black 1 is so simple it didn't occur to me.