Unless the dive mode turned off, the dive mode activates automatically when you dive deeper than 1.2 m (4 ft). However, you should switch to dive mode before diving to check battery condition, the display and so on.
Each time your Suunto D4f enters dive mode, a series of automatic checks are performed. All graphical display elements are turned ON, and the backlight and the beep are activated. Then the battery level is checked.
Before leaving on a dive trip, it is highly recommended that you switch to dive mode to make sure everything is functioning properly.
After the automatic checks, Suunto D4f enters surface mode. At this point, you should perform your manual checks before entering the water.