I've noticed that this ability no longer changes your defense after use. I think it just adds a superb "Physical Damage Taken -xx%". I was fighting ants today at Western Altepa (PLD60 Full AF). If my judgment is correct, the ants were hitting me for 45~ damage-- after I used Sentinel they only hit for 10~ damage.
Please post your observations of this ability. The more evidence there is the more facts we can conclude about this ability, which will allow paladins to more effectively use it.