How does Llengrath's Safeguard work?

[themango55]

"An insidious magical contingency activates when the caster hits 50% Stamina. He or she immediately knocks all immediately close enemies prone while gaining a Damage Threshold and Defense bonus for a short time.

 

+ 20 to all defense
+ 10 DT"

 

Does someone know how this is used and how it works? Does it autocast? Do you cast it at the start of an engagement and it last the rest of the engagement (then is wasted if unused)? Can you cast it out of combat and it last until you rest if not activated?