Most purchases are habitual. Customers repeat what they bought last time because deciding costs energy and repeating doesn't.
A promotion has to be strong enough to interrupt that default. That's why headline mega-prizes outperform another 10% off — the promise is memorable, the cost of ignoring it feels real.
The mechanic is the exchange rate: what behaviour do we want, and what is the smallest reward that reliably triggers it? Everything else is scaffolding.