[cuttysnark]

I was confused too, but I'm stubborn, eventually figured out:

Clicking "Draw Fast" creates a box with the label "a city skyline"—drawing in that box will render an image to the right as you draw.

Double-clicking the box that "Draw Fast" produces allows you to edit the tag which says "city skyline" by default—allowing you to change what it renders as you sketch.

[berryg]

And can you copy and/or save the generated image?

[cuttysnark]

I can't seem to find an explicit save or export option apart from File > Print. Using that, the Chrome print dialog, "More Settings" allows it to be opened in Preview or "Print Using System Dialog" where it can be saved to PDF.

Alternatively, I just screen captured the rendered image with MacOs "copy selected region to clipboard" command.