They break their orders into smaller pieces, and build (or rent from their brokers) algorithms to make their big orders harder to spot and less likely to move markets. John Arnold "spent millions of dollars developing a proprietary order-entry system to disguise and conceal strategies from external algorithms."
The "ideal" is that this would not be necessary. That this would provide no advantage, because markets would never try to spot "large intentions". Party B would not temporarily set a price that is less profitable for selling small numbers and more profitable for selling large numbers.
Markets don't need immediate higher-order feedback to be efficient.
And again, don't ask me how you would stop anyone.
If you have 10,000 shares that you want to sell, and you want to sell them all at a specific price, there is a simple mechanism to do that: enter a limit sell order.
Party B is then inferring things about supply/demand patterns. He is not tracking a specific entity. Party A meanwhile is doing everything in his power to hide his large intentions (trading across multiple venues, with different executors, at different times, at different sizes, etc). This natural adversarial relationship is what causes the price to go from 9 to 9.20 and it does not instantly jump, it is smooth. This is the mechanic for how that smoothness occurs and happens in real life and needn't add any impossible principles to the mix for it to occur.
In either case, Party A is unhappy about it jumping. They want it at 9 for as long as they can charge it, and Party B wants to maximize the average price they can sell it at (that is it is better for them to continue selling at 9.18 than to scare everyone off at 9.20).