You don't need to worry about where the sparseimage is, just save your files to the iDisk and keep them there. The sparseimage sync happens automatically, and you don't need to know it's there.
The best way to try it is to enable the iDisk sync and wait for it to complete (in the Finder there will be a small sync animation next to the iDisk icon in the sidebar). When it's finished, disconnect from the internet and you'll find you can still use the files on the iDisk, because you're using the sparseimage copy. Change a few files and save them, and then reconnect to the internet, and the iDisk sync to the server should take place. You can force a sync by clicking on the thingy next to the iDisk icon in the sidebar.