When Amazon observes a sudden sales spike for a day, they typically think of it as nothing spectacular because anyone can have sales spikes for a variety of reasons. The main reason the one or two-day product launches fall off rapidly from rank is that if you don't immediately start converting organically, Amazon recognizes that temporary spike as an outlier, and thus decreases your rank. Maintaining a steady sales velocity beyond one or two days will have a more significant blip on Amazon’s radar, particularly when you gain organic sales at full price after the launch period.