Flash sales compress the normal shopping decision into a much shorter window, and that compression is exactly what makes them effective at generating purchases that would not otherwise have been made. The discounts offered can be genuine, but the format itself works against careful decision-making unless a shopper has a plan going in, since there is rarely time to research an unfamiliar product properly once the sale is already live.
A short time window removes the natural pause that normally sits between seeing a product and deciding to buy it, and a fast-moving stock count adds a second layer of pressure on top of the discount itself. Together, these conditions push a decision that might otherwise take a day or two into a matter of minutes, which is precisely why unrelated items often end up in the same cart as the one thing actually being sought.
The most effective way to shop a flash sale well is to decide what to look for before it begins, rather than browsing the sale itself for inspiration.
For anything not already on the prepared list, a simple pause helps: closing the tab and returning after a few minutes with a clear head is usually enough to tell whether an item is a good addition or simply a response to the countdown. If the item is still wanted once the initial time pressure has faded, it was likely a reasonable purchase; if the appeal was mostly the ticking clock, skipping it rarely leads to regret.
Flash sales reward preparation more than speed. Shoppers who arrive with a list and a limit generally get the intended benefit of a genuine discount, while those who browse freely under time pressure are the ones most likely to end up with purchases they later question.