This demo emulates an "iambic keyer". A keyer (combined with a "paddle") makes sending Morse code easier than the traditional "straight key" which most people would imagine is used. Instead of having to precisely time the dits, dahs and the spaces in between a paddle/keyer combination does a lot of the timing for you, using a speed setting of your choice. Pushing the paddle one way sends a string of repeating dits and pushing the paddle the other way sends a string of dahs. An "iambic" keyer uses two paddles side by side and therefore lets you press both at once ("squeezing" them): in this mode dits and dahs will alternate.
This demo is configured to use the "Z" key for one paddle and the "X" key for the other. Which one sends a dit (left or right handed) can be changed. The message is typed out in the page as you send it, with unrecognisable characters shown with a "#". Hovering over a character in the message shows the dits and dahs for that character.