When the two documents to be compared are loaded into the comparison window, a comparison is automatically carried out if the Autostart Comparison command is toggled on (which is the default setting). In addition, comparisons can be explicitly started any number of times with the Start Comparison command.
When editing documents, they are compared dynamically as you type if the Compare while Editing command is toggled on.
You can run file comparisons using all three of the methods listed above; these methods are not mutually exclusive. If the Show Options Before Comparison command is toggled on, then the Comparison Options dialog box is displayed each time before the comparison is run (except when it is run dynamically by DiffDog while you edit). This is useful if you wish to run comparisons with different options.