How we change what others think, feel, believe and do
Threaten them with extreme action that will cause them significant discomfort.
If they do not comply with your demands, threaten to do something that will cause them significant trouble and pain, even if it also would cause you pain.
There are two dimensions that you can apply: the level of pain and how long it goes on for. A short, sharp shock is often better than a long campaign. However, if you cannot cause significant pain, a long war of attrition may be enough. A dripping tap wears away even the hardest stone.
If you don't give me what I want right now, the next thing you hear will be from my lawyer. I'm not kidding here: I'll sue you for everything you've got.
If we cannot agree on the right price for your company, I may just set up in competition with you and drive you out of business.
I won't! I won't!! I won't!!! And if you try to make me, I'll scream and scream and scream!!
When you threaten war or some other extreme action, you are demonstrating that you are prepared to go to any lengths to get your way. This lack of consistency with 'normal behavior' makes it difficult for the other person to predict what you will do and their consequent fear leads them to capitulate.
It is particularly scary when they realize that you are prepared to do battle even if the cost to you is high. This lack of rationality again makes you difficult to predict.
The notion of extreme action also gives a contrast between the loss of capitulation and the loss that the extreme actions would cause. In this case, even total capitulation may seem like a better option.