The Importance of Drain Theory in Economic History

The drain theory is that the wealth of South Asia and other parts colonized by Europeans was drained in order to help Europe develop economically. I think it's hard to imagine the peculiar economic development of the European continent without at the same time keeping in view what Europeans did in other parts of the world. Can you give an example about the knowledge? So for example, Europeans didn't really know how to manufacture cotton and what did they do in order to learn how tomanufacture cotton? They traveled to South Asia and they observed Indian origins in how they produced cotton textiles and they wrote these things down. It took them a very long time but eventually they succeed by the late 19th century.