Your dog responds in the training before you have given a command yourself?
Of course your dog also hears the instructions from your trainer and in the meantime know that you repeat that.
With a clear instruction set you avoid that, you hear the trainer directly and the command only comes to you, not the dog.
It is also nice for the trainer that he should not raise his voice but can talk on a normal or even whisper level.
The range of the WIS instruction set is at least 200 meters so more than enough to bridge every terrain.