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I mean, even if you knew 100 people who had $100,000 each and they simultaneously sold all they had, that combined would still only be 0.2% of the daily volume during a big drop.

Yes I agree, but we're not talking $100k when it comes to a lot of people, the deeper you get in to this world the more you will see how much some people invest, I agree that it's mainly firms and some VCs etc., but they aren't the only ones who have a defined effect.. in fact the snowball effect could be produced by very small percentage moves.