Mac OS: Hiding an Application When Switching to Another
Can I switch to another running application and hide the current application in one step?
Yes you can. While holding down the Option key, select the application you want to switch to by selecting the application from the Application menu. You can also click in any open window of the application you wish to switch to.
Having the Option key held down hides the current application, including any windows open in the Finder, and makes the application you selected active.
This feature is possible with any version of the Mac OS beginning with Mac OS 7.0.