is an overplay. Black should defend at before taking such actions. But of course then then white will probably play a keima at the top star point, to avoid .
is a probe to check black his response. Black can defend his 2 stones by playing at , but he can also play a hane at the 2-2 point.
If black answers with a hane then white will be glad to capture the 2 black stones.
If black protects his stones then white can play the forcing moves of , and . White does not have to worry about eyeshape anymore. So he is strong enough to start a fight and he can cut the black keima shape with and as seen in the diagram.