After Black 5, White a is answered by b. This is an example of oi-otoshi.
Black 3 makes miai of a and b for life.
Giving away both 1-2 points seldom represents the main line. Black easily makes two eyes at the circled points.
Black 1 is a wasted move. In comparison to the main line, he falls one move short in answer to White's placement at 2.
Black 1 fails: White 2 leaves a and b as miai to kill.
Black 1 expands the eyespace and again White answers at 2 to take away one eye. Next Black plays at what seems to be the vital point, but White has a nice answer up her sleeve. White 6 makes miai of a straight three big eye at a and a false eye at b.