What is often needed is a couple of strong loud barks just at the right moment. Many breeders teach their Kelpies to speak on command and keep them quiet the rest of the time. These are common terms used to explain a dog that works very slowly with precision and uses a strong staring technique to control the stock. This is more common when working a small group of sheep. Small numbers of sheep are extremely hard to control and many dogs cannot do it at all.
Sheep are naturally gregarious.
They like to stay in big groups and feel safe that way. When there are only a few sheep they get nervous and must be handled very carefully so as not to spook them. Arena Sheepdog Trials in Australia use three sheep for their test. This makes it very hard and only steady dogs that watch what they are doing can succeed in getting the sheep through the course. Three sheep was decided a long time ago as the minimum that could be worked with control.