Main line & variations: unkx80 and Dieter
If White connects at 6, Black 7 and 9 captures the three stones including 8 (White a, Black b and the result is connect and die).
White can not play this way because the marked black stone makes assures the second eye at the circled point.
The order of play is important here. If Black tries to get a stone at 7 first, then gives atari, White's shortage of liberties has bvanished. She plays 10 to make miai of cutting at a or saving the cutting stones at b.