how do you select the engine

[dax006]

I love the project! What I don't understand is how you hook it up to an engine (Selecting the engine in real time is really cool).

[jtiscione]

The engine is a web worker loaded from a separate JS file. They all work the same way- you send a UCI protocol command to it like "position startpos moves e2e4" using postMessage() and get a UCI response like "bestmove g8c6" or whatever in onMessage().
If you look in play.js you see how it's set up in a variable called "engine". If you select a different engine in the dropdown, the code near line 520 will shut down the current engine and set up the new one.