That's not the saving throw, that's the duration determinant.
The "saving throw" is the attacker's roll vs. your Ref, Will, Fort, etc made when he attacks.
Then, once we know whether you've saved or not (just like in 3E), duration of effect needs to be determined. Instead of rolling a set amount in advance (d4+1 rounds, or whatever), it's made a little more unpredictable by rolling each round to see if it has ended yet.
The "saving throw" is the attacker's roll vs. your Ref, Will, Fort, etc made when he attacks.
Then, once we know whether you've saved or not (just like in 3E), duration of effect needs to be determined. Instead of rolling a set amount in advance (d4+1 rounds, or whatever), it's made a little more unpredictable by rolling each round to see if it has ended yet.