Because the document is checked out, ndSync will turn User2’s changes into an unofficial version of the document and save that unofficial version to the server, and User2 will receive a bubble message about this. For a long as User2 keeps that document open, the changes User2 makes will be saved to that unofficial version. After User1 and User2 close out of their documents, ndSync will remove the unofficial version from the local computer (it will still have been saved to the server) and then sync down to the computer the updated, official version of the document, based on the changes made by User1.