Redirect fire modifier is getting filled and updates its duration when tagged enemies are dying, you don’t even have to kill them yourself.
Usually you will fill your redirect fire at the start of the encounter with max stacks, and then any tagged kill will just refresh duration of those max stacks and fill it with more if its not max. so you will have 12% (if you use 8 stacks which is recomended) damage increase to you an your team most of the time.
Only time you want to wait for max stacks during a fight is to tag boss, otherwise just tag everything you trying to kill and dont bother about the stacks amount.
Most of the time you will have 2 stacks available for tag, which is 8%, because of this: