If you interrupt the startup process by holding down the space bar to access the Extensions Manager, you can force your computer to restart immediately by holding down the command key while clicking the Extensions Manager's close box. The computer will restart and load the extensions that were selected when you clicked the close box.
This can be a useful procedure because some items like RAM Doubler, NOW Toolbox, and At Ease Startup require a full restart to load after being turned on in the Extensions Manager.