Assuming the hunter has 3/3 in the 'Exposed Weakness' talent, his ranged critical hits will have a 100% chance of proccing an Expose Weakness effect onto the target. The Expose Weakness proc increases ALL AP of attackers (both physical ranged and melee) against the target by 25% of the hunter's agility (the higher the agility, the better) for 7seconds.
This Expose Weakness debuff on the target applies to the whole raid unlike True Shot Aura which only applies to party members. Hence, crit rating and agility is of the utmost importance to the Survival hunter. More crit = more proc --> nearly never ending Expose Weakness.
For a raid, best to have a MM hunter / Surv hunter party combo. The MM Hunter and rest of the raid feed off the EW procs while the Surv hunter feeds off the MM Hunter's TSA to boost his AP.
Expose Weakness is a hunter survival talent and is also available as set bonus of Dragonstalker Armor.The 8 piece set bonus of the Dragonstalker Armor gives you a chance whenever you deal ranged damage to apply an Expose Weakness effect to the target. The Dragonstalker Amor Expose Weakness proc increases the Ranged Attack Power of all attackers against that target by 450 for 7 sec.