When is working a sheepdog like pushing a Supermarket trolley?
Take my word for it, driving sheep is like pushing a loaded supermarket trolley; the thing seems to have a mind of its own, and so do the sheep. If you push the trolley straight, it wants to go to one side or the other - likewise the sheep. If your trolley is determined to go to the left, you have to position yourself on its left-hand side and apply pressure diagonally to the right in order to keep it in a straight line. This is precisely what the dog has to do to balance the sheep.
The heavier the load in the trolley, the more determined it is to go the way it wants and the more pressure you have to apply. Likewise 'heavy' and 'light' sheep. Heavy sheep try to ignore the dog and go the way they want. Light sheep run away from the dog if it gets anywhere near - so for the heavy sheep, your dog must be strong-willed and able to push them hard but for light sheep he needs to stay well back and apply the lightest of pressure.
The supermarket trolley is also like a bunch of sheep when you want to turn. If you try to twist the trolley in the direction you want to turn, you'll find it almost impossible but if for instance, you want to make a turn to the left, simply move yourself to the right and push - this is much like a dog flanking sheep and then walking up on them. One vital difference between the trolley and sheep is that if we want to stop the trolley, we can pull back on it. Of course, we can't do this with our woolly friends, so let's imagine we can't pull back on the trolley. We now have basically two ways of stopping it - we must either put something in front of it (again, impractical in the middle of a field) or we must run around to the front of the trolley and push it backwards. This is the most natural instinct of any border collie sheepdog (not pushing supermarket trollies) and you don't usually need to train it.
If the dog's going to work sheep it will very quickly learn that, to stop the sheep, it must 'head' them. Of course, it won't actually lean on them in the way that we would stop a trolley, but it will apply pressure to the sheep by simply getting in front of them. Bizarre though it may sound, you would be wise to familiarise yourself with these points before you even attempt to take your dog to sheep - but remember to return the trolley to the supermarket!
The whole business of introducing the dog to sheep can be altogether more dignified and controllable if you and the dog establish who is in charge and what the procedure will be before you get there.
Rather than just taking the dog to the field and letting it go, we can introduce much more control by the use of a line. If you're going to use a long lead, make sure you wear gloves to prevent friction burns when you have to stop the dog and bear in mind that, unlikely though it is, there is a small possibility that a trailing cord or lead could snag on something and injure your dog. Don't use a trailing lead or cord unless you're prepared to take this risk.