I use conflict to bring out aggression, kind of like the handler unfairly correcting a strong dog and he got pissed of and bite the handler, but we direct the aggresion toward the helper instead.
Assuming the dog has a solid bark and hold, and not in the begining stage of b&h training, and that he was trained properly, balance in prey and defense.
I send him into the blind for the bark and hold, and have a long line hook to the prong colar, as he is doing his barking, I will give him a pop, and immediatly the decoy will crack the whip, the dog is thinking that I am doing this correctly but why I am being corrected, this created conflict and pissed him off, that will bring out the aggression and as the decoy crack the whip immediatly after the correction, the aggression is channel toward him instead of me the handler. You have to be careful that you and the decoy works in unity, otherwise the dog might turn around and bite you instead, and also not correcting too hard that might bring his drive down, after a few session you should be able to know how much pop to apply to bring out the most aggresion in the bark, on every session, decoy should let him bite and put up a good fight then let him win when the agression level is highest.
Good luck.