"You Think, You Blink" was a short lyric, that Bert was to sing just before entering the chalk painting (and starting the "Jolly Holiday" sequence) In the film, Mary Poppins. In the finalized movie, Dick Van Dyke simply recites the lyric instead of singing it.
You think, you blink, you do a double wink
You click your heels twice in the air
And quick as a tip you're there