Sorry it isn't more clear - by default, you get "non-scratch mode" until it's determined your GPU (hardware accel in browser) + CPU can handle the extra load. Once you opt in, localstorage remembers that preference until you opt out.
You can force scatch mode via http://wheelsofsteel.net/?scratch=1 - but without hardware accel (hardware + OS + driver support, I recommend Safari 5 or Chrome 12), rendering falls back on CPU use which will skyrocket and that will kill the audio processing bits.
I wrote a small book on the whole process (and performance caveats) on my main site as someone linked below. Numerous pics + videos too. ;)
