The real wordsTheory
The third source of trade
You have now met two bases for trade. Differences in the relative availability of labour, capital and natural resources, which is Heckscher-Ohlin. And economies of scale with product differentiation, which is chapter 6.
Trade based on dynamic technological differencesTrade that arises because technology itself changes over time at different speeds in different nations.
Two models cover it: the technological gap model and the product cycle model. Because time enters both in a fundamental way, they are dynamic extensions of the static H-O model.