Marxist Economics: Understanding Capitalist Exploitation through "speed up" or increase in intensity
Though I'm certainly no expert in Marxist economic theory, I think he's presenting it as an "if you can figure out a way of speeding workers up, then you can generate additional surplus from their efforts. Effectively, this is just saying "say you're Ford, and have teams of 7 working on assembly lines that generate 4 cars per hour. Suppose there is some way to increase the intensity of labor, by getting them to work faster and produce 5 cars per hour, and not changing the number of assembly lines or workers per line. Then, if you can hold their wages fixed, you can generate more surplus for the capitalist."

(I'd imagine a historical example might be the semi-mechanized looms of the English Industrial Revolution, where the speed of the mechanisms was dictated by the machinery, and could therefore be bumped up by a factory owner.)

I don't think there's a single coherent mathematical argument explaining why this speed would be increased, just highlighting the different ways the ratio of interest can change.