We currently have (nominal) Chihuahuas but my favourite breed is bull terriers. Bullies are the court jesters of the dog world.
Minder, as my mothers dog was called was built like a brick sh....er, outhouse. When my mother took her to training classes the first lesson was to tie the dog to a fence and walk away about 20 feet so the dog apparently learns not to bark and whine and that its owner will come back. All the other dogs remained at the fence and barked or carried on - not Minder. When mum turned around there was Minder, right behind her, having snapped the leash!
Recommended method to get the dog to drop was to say the word "Drop" loudly and firmly, gesture in front of the dog and put your foot on its back to push it to the ground. With Minder you might as well have taught the coffee table to drop - the effect was about the same.
She was five and a half stone of powerful muscle , yet the gentlest dog I ever knew.
GG