Each successive time you target the same body part in V.A.T.S., your chance-to-hit raises by +5%. So, if your first to-hit was 40%, 2nd would be 45%, 50%, etc. This can lead to very easy head-shots when teamed with other to-hit % boosters (Sniper, Commando, etc.)
“Stay focused! This focus only stops when you run out of Action Points, so bolster APs to further enhance this perk. As you'll know how many APs it takes to fire off the bullets from your weapon, you should surmise that a heavier weapon, or one that fires slower and more damaging projectiles, is more likely to use up your Action Points more quickly and lessen the effectiveness of this perk. So play around with your favourite weapons to see which ones are most effective.”— Fallout 4 Vault Dweller's Survival Guide description
Concentrated Fire is added to the game with the Fallout: New California expansion. While in a fight, the player character can discard the perk card in order to change two of the VATS dice to match the third die of their choice.
Unlike other perks that give bonuses to your accuracy in V.A.T.S., such as Commando, Gunslinger and Sniper, the bonuses gained from Concentrated Fire are additive instead of multiplicative.
This perk is more effective with weapons that use few Action Points, so that you can fire many times and gain larger bonuses.
As the perk says, the bonuses are per-ATTACK, not per-HIT. So when queuing up shots, each subsequent shot will have the increased accuracy reflected in VATS immediately. Similarly, regardless of whether any previous shots hit or not, the nth shot will deal 1.15n-1 damage at Rank 2 and 1.2n-1 damage at Rank 3, for up to 15x damage on the last shot!
Accuracy can't be higher than 95%.
The final accuracy bonus for the last shot made does not apply to all of the shots made. The accuracy of each shot is based on the accuracy it had at the time you queued the shot.
(Fallout: New Vegas) Due to some perks and traits that can lower AP cost (Fast Shot, Math Wrath, etc.), using the AP reducing perks along with a weapon that has low AP like the All-American can result in very large bonuses for queued shots.