
Mort the Cleverly Named |

I'm honestly not entirely sure what you are asking.
Critical Focus gives a +4 bonus on attack rolls to confirm critical hits.
So when you roll a critical threat (natural 18+ for a Falchion, natural 15+ for a Keen Falchion), then you get an extra +4 bonus on your roll to confirm that critical hit. So, say your normal attack with the Falchion was +10, you would roll with a bonus of +14 when confirming the critical hit.
I hope that explains it.

Mort the Cleverly Named |

Ah! I think I understand now.
When confirming a critical, you only need to hit the opponent's AC with the confirmation attack roll, not roll within the "critical threat range" a second time. Critical Focus is a bonus to that attack roll.
Lets say you have an attack bonus of +10 and are attacking an opponent with an AC of 20. You roll a critical threat, then roll again to confirm. You would normally need a 10+ roll to hit, but Critical Focus means that your bonus is now +14 and you will confirm the critical on a 6+ instead.