After a double kakari upon its hoshi stone, AlphaGo shows us some new ideas. The attach, hane , extend continuation shown below ( to ) is absolutely standard. For move 4 there are other options, but here we talk only about the second attachment.
Master makes this solid connection, allowing the push and cut threat inherent with a
Zero instead choose whereupon there are options to play a which is big for both, and b which would remove aji for white.