With black a or b a simple response is white pulling back at c. Black b unsettles white while defending the corner whereas a lets white easily turn the probe into an extra long extension. With both a and b white can simply tenuki leaving a lot of aji, or sacrifice
, for this reason iron pillars are a common defense.Evaluating the simple response is important, it is not great if the opponent can gain an edge by straightforward solid play. In this case pushing along a gives white a base while connecting with
and leaves the corner open. With b there is no sliding into the corner, no base for white and a easy split of and because of the connection underneath.