Basically, that third world countries/global south (called the periphery by Amin) are not underdeveloped, but what the imperial core/Center nations economies are built on though exploitation. The economies of periphery countries are permanently structured by imperialism to serve the needs of the imperial core nations.

An important part here is that counties in the periphery cannot catch up in development with the imperial core countries in the system of global capitalism, because of monopolies held by the imperial core nations, and inherent polarisation between the core and periphery. Monopolies such as nuclear weapons, communication systems, technology, global financial systems and access to natural resources. These monopolies are maintained by organisation such as the IMF and NATO.

Thus the way to develop is to "de link" from imperial core countries and prioritise domestic development. Obviously due to globalisation, delinking 100% from imperial nations is not possible and economic suicide. If I remember correctly, Amin stated that de linking by 70% is the best most periphery countries can hope to achieve as an end goal in the current system. In 2017, he considered China 50% de linked. For interests sake, South Africa was 0% lol.

This is obviously a big oversimplification, I'm missing out on a lot of stuff, also I'm new to it so I could be summarising wrong.