For some customizations you do need to stop the form onsave event, perform some business logic and continue the save event. An example would be that in specific conditions are met when a record is saved, then a popup will need to be shown. After filling in data in the popup and pressing a continue button on the popup, then the save operation would need to continue. The SDK helps in this situation. Look at the following page for more information.
On that page you will see that you can stop the onsave event by setting the 'event.returnValue = false'. Don't forget to follow that line with a 'return false'. This will cause the save procedure to stop right at that point. Otherwise statements after that will still be executed.
Function: crmForm.SubmitCrmForm(59, true, true, false);