Tap-and-hold to do this, two fingers for that, swipe and shake for the other. Phew. Fortunately, not knowing the touch gestures listed on this page won’t impede daily use of the app.
These gestures are hard to guess, and there aren’t any visual clues in the app to guide users. I only discovered this page because I follow a couple of Google blogs: the overwhelming majority of iOS users won’t be aware that Google has published these tips.
I think Google are fine this time, but it highlights something I hope developers of human interfaces keep in mind as touch interfaces become even more prevalent: the very fine line between discoverability and usability.
A good rule of thumb is that the user should be able to figure out how to use an app just by looking at it.