To split your code, simply insert calls to the method GWT.runAsync at the places where you want the program to be able to pause for downloading more code. These locations are called split points.
A call to GWT.runAsync is just like a call to register any other event handler. The only difference is that the event being handled is somewhat unusual. Instead of being a mouse-click event or key-press event, the event is that the necessary code has downloaded for execution to proceed.