As stated, it could be anything. Space Elves are all the same anyway.
Perhaps the regiment fights Eldar a lot, and the fighting styles are similar.
Perhaps it was a "Thirty Days of Night" Scenario, where the IG/PDF are stranded on a planet, cut off by warp turbulence or the shadow of a nearby gas giant or something, and then subsequently tortured by dozens of raids over a prolonged period of time until the hardbitten survivors become attuned with an almost sixth sense.
Perhaps the regiment takes part in experimental drugs testing or uses banned or controlled substances or is being monitered, guinea-pig style by an Inquisitor while fed drugs that boost reaction time, stamina and aggression, and it just so happens that in this particular battle, they're fighting a Dark Eldar raiding force that got more than they bargained for.
Maybe the regiment consists entirely of Ninjas who were orphaned by Dark Eldar slavers. With a score to settle, these black clad night assassins fight against their hated parent-murderers with laser shurikens and untold ferocity.