The RR will last for 0.3s
I think its easiest to understand wiht an example.
Target has 0% fire res
I use OFF and apply -100% RR for a 0.2s freeze duration.
I apply 300 burn damage over 3s on the target during this 0.2s window.
Because the target’s res is -100%, it will take 600 burn damage over 3s.
This amplified damage will persist even after the target becomes unfrozen
EDIT: it’s the same concept as extending the freeze duration on a target with lowered freeze RR, except we are talking about DoT’s here