The Explain Plan can be executed using the menu item 'Explain Plan' or using the key combination Ctrl+E:
The executing of 'Explain Plan' ensures the filling of the tab 'Explain Plan' with the rows which are completed by the 'Explain Plan' of Oracle:
In the tab, the way the query will be executed by Oracle is shown in tree form.
With this the to be executed actions are in order of the ID, so from a branch with the deepest level back to the parent level and back to a related level; this is called 'Processing in Order'.