In a patrol dog with high hunt drive and fight drive and prey drive in balance, the dog will find the "bad guy" and bark and hold.
This bark and hold comes from actual hunting of prey for food. The dog finds the animal which freezes (deer or elk or whatever) and the dog barks and "holds" the prey in that position. The barking is to signal other members of the pack that prey has been found and cornered and they need to come help "harvest" dinner. The prey animal finds the pack too much pressure and then moves, at which point the pack moves in and bites/kills the prey. A different dog may actually do the killing than the dog who hunted the prey and alerted the rest of the dogs to come. Unlike wolves, dogs leadership changes with the situation and the dog best at the current job at hand is the one that leads. You can see this in Cape Hunting Dogs in Africa.
Back to man trailing and the patrol dog.... the minute the "bad guy" moves, the dog will go in for the bite. The bark and hold on the bad guy is to call his Partner (the police handler) for help in apprehending the person. A bite at the end of the hunt and a fight in a good patrol dog can be a great reward. So, we utilize the natural genetic ability of the dog to do police work.
In some cases, a suspect will run and the dog will not grab an arm or leg, but instead will chase and launch at the person, hitting them really HARD in the upper middle of the back and take the person off of their feet. This is also a genetic behavior and comes from Herding lines (the dog will make a full, calm grip of the sheep and push the sheep to the ground to control it.. and does not damage the sheep). Of course, when the dog knocks a bad guy down, the dog may end up in a fight still and need to bite. A truly good patrol dog is a true joy to see work.. and they LOVE the work.
So, for some dogs, finding someone who will give them food is the reward they need to keep working. For other dogs finding someone who will give the dog a bite is more rewarding. The latter is likely not a good choice for SAR. You need to know your dog.