"Push your dog's rear down" the teacher belted out. We all reached down and pushed and pushed and pushed. Some of the dogs buckled under pressure while most resisted, pushing back. This is the old conventional way to teach a dog to sit; just push them into it. Unfortunately whoever decided that this was the way to do it was sadly mistaken. Dog mechanics is as such; you push, they push back. So just imagine what happens when you attempt to push them into a sit? Yep, tons of pressure on those hips and you can end up in quite the struggle; depending on how big and strong your dog is.
Same goes for a down; start pushing and you may find yourself in the middle of a wrestling match. Our natural instinct to physically maneuver a dog into a particular position goes against canine mechanics. I remember in my conventional training class many, many years ago when owners would be literally on top of their dog trying to get them into a down position, pretty crazy.
Mechanics: the branch of physics that deal with the action of forces on bodies and with motion, comprised of kinetics, statics and kinematics.
Pull, they pull; push, they push.
The funny thing is that dogs are sooo very easy to put into different body positions when you lure them with either a food treat or some sort of toy. I have taught tiny little 6 week old puppies to sit within minutes, just with a cheerio. It is much easier to move dogs if you don't try to man-handle them. Moving them just by training position and teaching the point tool is really much less work.
Many dogs who have never been taught about moving around, sitting, downing or staying get very anxious when you begin to move them physically. The more they struggle the firmer we get which then makes them struggle even more; it is a lose, lose situation.
If you do have to physically need to move your dog; do so with the least amount of touch possible. Do not rely on the collar; that too can backfire. Learn how to move and position your dog without having to muscle them around; it is much easier after all.