Not the person, but for a leftcom (and even more strictly applied ML theory) imperialism is when you have a different 'economic zone' that applies to another country. Essentially it is when you have 'rules for thee but not for me'. The classical mercantile imperial model develops the extractive industries of a colony, while the actual processing industry exists at home. You use cheap labor in the colony to extract the materials, refine them back at home, and then sell them back to the colony, rather than developing the colonies ability to feed and educate itself, and then also develop it's means of production. Instead, MoP is developed mostly for the benefit of the colonizer. This is also maintained through different labor laws applying to these different zones, which are then enforced with extreme prejudice, which is the hallmark of imperialism. This is laid out in Lenin's 'Imperialism'.

Now, what leftcoms usually miss imo is the part where it is an outside force that is enforcing this through violence. For example, many leftcoms believe that what China is doing is 'imperialism' especially since it often relies on creating differently taxes economic zones within areas of Africa (or more pedantically saying that the communist party elites have sold out the proletariat of China to the highest bidder on the international market). While there is an argument to be made there, I think the important thing to note is that China does not (or at least hasn't yet) enforced their deals in Africa with military force, but instead through long term structural development deals where they develop the infrastructure of country on the cheap (because excess construction labor is something they have in abundance) in exchange for these economic zones where their population can immigrate to and work. It ultimately a win-win imo, and looks fundamentally different than previous versions of imperialism. Additionally, despite 'selling out their proletariat' the Chinese Communist Party has demonstratively not given up their control over the means of production and it's development. Wether you agree with it or not, what is happening to them is not 'imperialism', it's something else.

Capitalist development (i.e. investment in equipment and materials) is where the tendency for the rate of profit to fall comes from; as the amount of fixed capital increases in the system (machines, infrastructure, education, etc) the less labor is needed to produce the same quantity of commodities. As labor is the source of all value, reducing the labor inputs actually reduces the value of the physical output of the system.

Underdevelopment is a tactic whereby this process is undercut by deploying counteracting factors to reduce the rate at which profit falls, chiefly superexploitation of the workers and immiseration of the population. Instead of investing in fixed capital to extract resources and reduce the need for human labor (i.e. development), imperialism sidesteps the problem by using military and police instead.

Within the imperial core the tendency of the rate of profit to fall is also counteracted because the resource colony cheapens the elements of constant capital, industrial inputs, and consumer goods. Even better, underdevelopment increases migration away from imperialized nations towards the core, increasing the surplus army of labor and allowing for the creation of internal colonies of superexploited workers within the imperial core itself! This allows for further capitalist development within the imperial core without reducing the rate of profit because, while labor inputs decrease for the core workers, they remain constant or increase for imperialized workers.

... at least, that's as much as I understand. Still need to read more theory.