Markets are more complex than the simple rule of buying X shares brings the stock price up so much and selling X shares pulls the stock price down so much. Instead, buyers and sellers are affected by expectations as well as disinformation.
Take, as an example, the inability in recent days for TSLA to climb above $200. Market makers and hedge funds have been shorting TSLA as necessary (capping) to keep TSLA below 200 when it is quickly heading for a higher price right after market open. If you were planning to sell some Tesla and you saw TSLA moving quickly upwards, your tendency, if you wished to maximize profits, would be to hold off on selling until TSLA looked closer to a top. What the capping through short-selling does is create an artificial top for the day that would not otherwise be present with normal market dynamics. The seller sees TSLA bounce off 198 and head down to 197. He decides this is the best TSLA will do for the day and sells here. His selling may contribute to a further dip in the stock price. Meanwhile, there are buyers waiting on the sidelines for either a deep dip or a strong increase which they don't want to be late buying into. When TSLA bounces off 198 and settles lower, these sideline buyers decide there's no reason to invest in TSLA this day because the stock is clearly not going to run much higher than it already is. They stay on the sidelines and fail to put upward pressure on the stock.
Of course eventually the cap fails. We saw such a failure in recent days when a known resistance point of TSLA, 167, was breached. Many buyers were aware of the resistance point and when TSLA broke through 167 you saw strong buying take it much higher. A quickly rising TSLA changes both buyer and seller expectations with sellers holding off selling until a higher equilibrium price is reached and buyers accelerating buying so as to catch the run-up early. It's all about psychology. The short-sellers who capped can slowly buy back throughout the day if they have succeeded in holding the cap, and they can finish with buying to cover during the 4pm closing cross when that buying has negligible affect upon the day's stock price and even less affect upon the inclinations of buyers and sellers going forward.
Shorting a stock during the day and then covering during the 4pm closing cross is just one technique for holding the stock price back. There's spoofing, as well, where hedgies and MMs may put in large sell orders above a certain price without the intention of actually selling. If the stock price gets too close to this selling price, they will pull the order before it is executed. What the spoofing looks like to an advanced seller or buyer with access to the TSLA order book is that there's a price above which TSLA will not rise because of the abundance of sell orders just above that price. Sellers decide that since TSLA won't rise above that price, they'll sell below. Buyers see TSLA unable to climb above that price and figure there's no more upside for the day (best to hold off buying until another day).
I'm sure
@Artful Dodger and others could expand upon these observations, but the bottom line is that not all buying and not all selling affects the stock price equally. That buying or selling that changes the expectations of the market clearly has a larger affect on the stock price than trading that doesn't.
Hoping this explanation has been helpful.
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