I reward my pup with a tug when he does a behavior correctly. Being a young pup, I play with him for a minute or two before moving on to another behavior. What I like to tell people (and something that was told to me by a trainer long,long ago) is you have 4 seconds from the time the dog does the behavior correctly to reward him before he thinks he's just getting a treat/praise/tug for the hell of it. With EDDs, NDDs, and some dual-purpose PSDs, a ball/tug is given IMMEDIATELY when a correct signal (bark, sit, down, touch, etc.) is given by the dog to indicate the target scent (narcotics, etc).