Before I started to train dogs professionally, I worked with horses. Anyone who works with horses quickly learns about a behavioural phenomenon known as the opposition reflex. It's that (pesky) habit of horses to push against a force: try to push a horse who hasn't been trained to yield to pressure, and he'll push more of his weight against your hand.
When I started having to explain certain behaviours to dog owners, I transferred the concept of the opposition reflex from horses to dogs. I knew that it wasn't a reflex by definition, as one requisite for a movement to be considered a reflex is its involuntary nature, but it was a really easy way to explain why animals inherently resist pressure, and therefore why you shouldn't attempt to train a dog by pushing and pulling the animal into position. With horses, this isn't a particularly common technique for obvious reasons, but with dogs, it very much is.
I recently read an article that I can't find anymore, but it brought to light the fallacy of the opposition reflex. It was very well written, delving into where the concept of the opposition reflex originated and how it's not actually a reflex.
The more I read, the more I realized I needed to stop using that term. Not only is it not scientifically accurate, it also doesn't help explain anything in terms of overcoming the problems associated with the animal's opposition to pressure.
All of this is likely irrelevant for most dog owners and handlers, but not so for dog trainers and behaviourists. The difference between a voluntary behaviour and a reflex should be clear, and semantics matters: calling opposition to coercion a reflex makes it almost sound like it can't be helped.
But what of the actual tendency of animals (including humans) to push against pressure? It's a very real behaviour, and important in animal training. Not all dogs exhibit it every time pressure is applied to their bodies, but opposing coercion is very useful. Think of when you try to regain your balance if you're pushed: you have a natural tendency to not just let yourself fall on the floor or be moved by an external force. Push a dog to the ground to get them into a down position, and you'll likely experience the same thing: he'll push against your hand as opposed to just letting himself plop to the floor. It's not a reflex, it's a voluntary behaviour. If the dog in question is rather small, it may not feel like very much, but it's still there!
As for the opposition reflex being responsible for leash-pulling, I've never bought that dogs pull because of pressure on their body (via a collar or body harness). They pull because they're tethered to a slow biped, and either haven't been taught to walk on a loose leash, or are more motivated to pull. That's not opposing coercion, that's just trying to get from point A to point B as fast a possible while being anchored to something slowing you down.
I propose a change of terminology: opposition behaviour, or, more accurately, opposition to coercion.
There! That's better.