Attacks made with the Precision(X) special rule are allocated by the attacking player, not the defender.
When any hit test is made for an attack with the Precision(X) special rule, if the result of the dice roll, before any modifiers are applied, is equal to or greater than the value of X attached to this variant of the Precision(X) special rule, then if a hit is inflicted by that roll, that hit becomes a Precision Hit.
These Precision Hits must form a separate fire group or strike group. Wound tests are mad as normal for the Precision Hits and cause Precision Wounds. In step 8 of the Shooting Attack process or Step 7 of the Initiative Step sequence, when selecting a target model for a fire group or strike group that is made up of Precision Wounds, the target model is not selected using the normal rules. Instead the attacking player chooses which model in the target unit will be the target model, and may select any model that is part of that unit.
Note that models engaged in a challenge cannot use this special rule to allocate hits to any model other than the enemy model that is engaged in the same challenge and that any attacks which are made as Snap Shots, or for weapons or attacks that have either the Blast(X), Barrage(X), Template or Reaping Blow(X) special rules, may never trigger the Precision(X) special rule no matter what the result of the hit test is.