The big push theory argues that economic development requires a minimum level of comprehensive investment in mutually supporting industries to take advantage of economies of scale and externalities. It identifies three types of indivisibilities - in production, demand, and savings - that must be overcome through a large investment package rather than gradual increases. Social overhead capital, like infrastructure, requires huge initial investments but leads to lower costs and indirect contributions to development over the long term. Underdeveloped countries face challenges achieving the necessary savings levels for a big push and must rely on outside sources.