The price of a stock [or the cumulative price of many stocks, which form the overall price of an index] is based purely on the latest price change.
If I own shares in an airline, and that airline shuts down operations due to, say, a global pandemic, I will no longer expect to receive the 'old price' when selling them. So I will lower my expectations. Likewise buyers will not offer the 'old price', they will only be willing to pay a lower amount.
When a single trade happens at the new price, that becomes the price, period.
Usually high volumes of trades are needed to shift price, without abnormal events shifting expected value of the shares. So if 1,000 shares are on offer for $100, and 500 shares on offer for $101, then 1,000 shares would need to be bought before the $101 price is the lowest available price, and then another 500 shares would need to be sold before the price rises again.
It seems you are using an attempt at 'technical analysis' to determine how to 'time' the market. This is incredibly risky. I would further caution you to never invest in something you do not fully understand - asking questions like this is a great way to increase your understanding, but always be sure to ask such questions before you put your money at risk, and not after.