Left-Click to drag the scene around. Right-Click to rotate the camera. Zoom in and out with the mouse-wheel.
Only works in browsers that support the canvas element, Chrome does the best job. Opera is actually the fastest, but for some reason the results are very inconsistent. Safari works well, but the preview is sluggish. Firefox is slow but stable and InternetExplorer wont work at all.
If your browser supports WebWorkers, then try setting the Worker-Dropdowns. Rendering time is decreased mainly for settings requiring a lot of CPU power. The preview generally got slower in my tests as the setup of the webworkers requires more than the actual rendering time.