Once you press F12, you enter the incremental search mode. Once in the incremental search mode:
F12 will search again for the current string, or it will load the current string with the previous one if the current string is empty.
Shift+F12 will search backwards.
Backspace reduces the search string.
Esc will cancel and return to the initial position.
Enter will stop and leave the selection at its current position.
Any key that maps to a command will exit the incremental search mode and leave the selection at its current position, and then the command will execute.
Any other simple key is added to the current search pattern, which is displayed at the bottom in the status bar.
The search buffer is left with the last successful search pattern.
The incremental search pattern is not case sensitive, unless you type an uppercase character.