I think it was basically the idea of synergy. Where was that?
You're probably thinking of Ch 13: Cooperation. Although it's not as simple as more cooperation = more efficiency, so depending on what you're investigating you might want to read all of Part 4 (Ch 12-15).
Here's the short of it. Cooperation cannot be thought of as in itself and transhistorical. The mode of production becomes more developed, simple cooperation moves on to manufacture and then to machinery and the factory. However so long as capitalist employment of machinery for commodity production is the dominant mode of production laborers effectively become slaves of the factory.
... Machinery, considered alone, shortens the hours of labour, but, when in the service of capital, lengthens them; since in itself it lightens labour, but when employed by capital, heightens the intensity of labour; since in itself it is a victory of man over the forces of Nature, but in the hands of capital, makes man the slave of those forces; since in itself it increases the wealth of the producers, but in the hands of capital, makes them paupers...
-Capital Vol 1
I have big dumb guy action movie energy and always loved where he describes the factory as like a lovecraftian demonic machine god.
An organised system of machines, to which motion is communicated by the transmitting mechanism from a central automaton, is the most developed form of production by machinery. Here we have, in the place of the isolated machine, a mechanical monster whose body fills whole factories, and whose demon power, at first veiled under the slow and measured motions of his giant limbs, at length breaks out into the fast and furious whirl of his countless working organs.
Goddamn this rules. Really gets at the raw excess of productive power the industrial revolution creates.
You might also be thinking of Proudhon's concept of Collective (Labour) Force.
