'Focus' is a DIY solution to any problem, or your quintessential 'oh shit' button. Valid uses of Focus include enhancing an existing Ability, solving a difficult problem, predicting danger well in advance, or avoiding significant harm. Think of it like rolling a natural 20: use of Focus provides guaranteed success, unless the opponent uses their own Focus to counter.
Focus can be used any number of times in a post/round, but once you run out, that's it until the end of the current scene/conflict.
When using Focus offensively with the goal of significantly harming your opponent, it is a common courtesy to get their consent before posting. If the significant harm will cause an ongoing problem for your opponent for the remainder of the conflict, such as a broken leg or a Debuff with the Ongoing modifier, getting their consent is mandatory unless you use the Focus at the end of your post, thereby giving them a chance to respond on their own terms.
If Focus is used to enhance an Ability used on yourself or an ally that has the the Ongoing modifier, the resulting effects last only until the end of your opponent’s next post (e.g., if you use Focus to enhance your Ongoing Speed Ability, the speed boost is active until the end of the post immediately following yours). As always, we ask that people try not to min-max by using Focus right at the beginning of every post to maximize the benefit, but as long as it makes narrative sense we won't grouse.
Focus also grants a certain degree of "narrative freedom" if you would prefer. For example, if Bowser wants to enhance his Bowser Punch to create a shockwave that knocks down and stuns a group of enemies, even if his normal Bowser Punch doesn't have ranks of Debuff or Affects Multiple, that is a perfectly valid use of Focus. While narrative freedom is all well and good, keep in mind that your use of Focus has to make narrative sense (e.g., your basic dagger Ability should not be carving through dense plate armor just because you used Focus).
Focus is not additive. Only one point of Focus can be used to enhance an action or Ability. This preserves the equalizing factor of Focus that keeps high level characters from having a profound advantage over low level characters. You may use Focus to enhance the same action or Ability multiple times, but not in the same post.
Finally, Focus operates under the assumption that all characters have a base level of strength, agility, coordination, and stamina/endurance (even if they are the equivalent of rank 0). That means any character can use Focus to pull off an incredible dodge, an unlikely eagle-eye shot, tank a stronger hit than usual, or hit much harder than usual. This also covers weapon proficiencies; if your character has never used a bow and arrow, use of Focus still grants the necessary level of proficiency to pull off that crazy shot. However, keep in mind that plausibility and immersion are important factors of storytelling.