Focus rule is designed to invite more dynamic and changing situations into battles. It's very simple.
A character may Focus during the battle on one task and receive +10 bonus to Test it needs to pass to fullfill it. If it's an Attack, it's target should be strictly specified. If it's using a skill, it should be determined what does the character want to achieve and whether there is other character involved in it (focusing on more than one character at a time is impossible)*.
Until Focusing character's next turn he or she needs to pass Awareness Test to notice all attacks coming at him/her, with the exception of attacks returned by it's target. If the Test is failed, the Focusing character counts as Surprised.
It's designed to add some depth to planning encounters and making them more sneakylike. Besides it makes real life tactics possible to use in the game.