Confuse about Call-on traits

I have a character with the trait Driven, as a call-on for Strategy. (He’s a very driven general, determined to lead his army to victory.) Mechanically, what does the call-on do?

You can use a Call-On trait once per session, to do one of two things - break a tied test in your favour (so you win, instead of tying), or to re-roll all of the failed dice.