That's what I believe:
Cap haste, land high dmg SA/TA and you'll occasionally get an increase. The further past your gear/atma #, the harder/less likely to gain.The effectiveness of the THF job trait "Treasure Hunter" now occasionally increases with each melee attack landed. Further, damage dealt with either the "Sneak Attack" or "Trick Attack" ability will have a greater likelihood of enhancing this trait.
XI Wiki


