Karl Knechtel: I didn't read all variations, but I'm pretty sure this simple move is the key. a is an "asymmetrical" shared liberty: White's play there does not cost her a own string a liberty, but Black's does. Since Black must either approach (i.e. connect at b) or block White at the top first in order to capture (i.e. filling the points to the left of White's string without either a or b is simply self-atari), White effectively gains a liberty in the capturing race and is ahead 6 to 5.