I emboldened a sentence that suggests Foucault might not have explained "boomerang" clearly.
Rather than merely highlighting the history through which European powers had colonised the world, however, Foucault’s approach was more novel. Instead, he explored how the formation of the colonies had involved a series of political, social, legal and geographical experiments which were then actually often bought back to the West in what Foucault – drawing possibly on Hannah Arendt’s famous work on totalitarianism – called ‘boomerang effects’. ‘It should never be forgotten,’ Foucault said:
_“that while colonization, with its techniques and its political and juridical weapons, obviously transported European models to other continents, it also had a considerable boomerang effect on the mechanisms of power in the West, and on the apparatuses, institutions, and techniques of power. A whole series of colonial models was brought back to the West, and the result was that the West could practice something resembling colonization, or an internal colonialism, on itself”
Such ‘boomerang effects’ centred on ordering the life of populations at home and abroad – what Foucault called ‘biopower’ and ‘biopolitics’ – rather than on protecting sovereign territory per se. Foucault did little to elucidate these in detail, and rarely touched on colonialism or postcolonialism again. However, his notion of colonial boomerang effects is powerful because it points beyond traditional ideas of colonisation toward a two-way process in the flow of ideas, techniques and practices of power between metropolitan heartlands of colonial powers and the spaces of colonised peripheries. Such a perspective reveals, for example, that Europe’s imperial cities were much more than the beneficiaries and control points organising explicitly ‘colonial’ economic techniques of plunder and dispossession through shipping, plantations, mining, oil extraction or slavery. They were also much more than a product of the economic booms that came with the processing and manufacturing of resources extracted from the colonies.