Imagine S&P goes -5% and so, technically, should VOO. But VOO is a stock itself - subject to supply and demand. The market could just happen to demand many VOOs and push its price up.
Why doesn't this happen?
financial institutions purchase and redeem ETF shares directly from the ETF, but only in large blocks (such as 50,000 shares), called creation units. Purchases and redemptions of the creation units generally are in kind, with the institutional investor contributing or receiving a basket of securities of the same type and proportion held by the ETF
The ability to purchase and redeem creation units gives ETFs an arbitrage mechanism intended to minimize the potential deviation between the market price and the net asset value of ETF shares.
If there is strong investor demand for an ETF, its share price will temporarily rise above its net asset value per share, giving arbitrageurs an incentive to purchase additional creation units from the ETF and sell the component ETF shares in the open market. The additional supply of ETF shares reduces the market price per share, generally eliminating the premium over net asset value. A similar process applies when there is weak demand for an ETF: its shares trade at a discount from net asset value.
What this is saying is that if you own the same basket of stocks as the ETF is made up of, then you can turn in those stocks to the fund, and get shares of the fund in return. So if the S&P goes down 5% but the ETF doesn't, then you can buy the stock for 5% less, create ETF shares from them, and then sell those ETF shares at the full price, netting a ~5% profit. This process will then create more supply of ETF, which drives the price down.