It's reflected all projectiles for quite some time, to include the basic attacks from bows. I would say time your power attacks and Venom Arrows for the timing of the buff, but with the problems with lag in PvP, it's just as likely to get you killed either way.

Suffice it to say stick to non-projectiles while you see the buff active, Bombard is a good one, and play keep-away defense until the DK burns out. The class doesn't have much in the way of Magicka sustain unless they're using light armor and Magicka Furnace. Either way the Light Armor spec needs to close into melee to be effective and if you can keep your distance with Bombard and back peddling (and s touch of Magnum Shot...be sure to hold block after you use it or during use to prevent the self kick back if the situation calls for that) you can wear them down for the kill. They have limited dodge and block capabilities as a LA spec too so use that to your advantage.

Don't panic, keep your head, pay attention, and you can out play the skill. Timing your power attacks and immediately chasing them with a Venom Arrow are your best friends in a relatively lag free fight.