Backpressure is an implicit signal from consumer to producer that there is not enough capacity. It propagates backwards from flow like water in a network of pipes.
In their example, the conveyor belt keeps adding chocolates because there is no backpressure. The effect is that Lucy engages in load shedding, and the producer of the chocolates has no say or insight into what happens when that load has to be shed. If there was backpressure, the producer gets to choose what happens.
Mozilla's documentation[1] similarly defines backpressure as "the process by which a single stream or a pipe chain regulates the speed of reading/writing".
The article confuses backpressure (the signal used for regulation of the flow) with the reason backpressure is needed (producers and consumers working at different speeds). It should be fairly clear from the metaphor, I would have thought: With a pipe of unbounded size there is no pressure. The pressure builds up in a fixed-size pipe when consumer is slower than producer, which in turn slows down the producer. (Or the pipe explodes, or springs a leak and has to drop data on the ground.)
[0] https://streams.spec.whatwg.org/#pipe-chains
[1] https://developer.mozilla.org/en-US/docs/Web/API/Streams_API...
> You could have back-pressure in the most optimized system.
Most batch processing systems. But you don't want back pressure in interactive or real-time systems, like graphics, gaming, audio, cars, planes, or even just real time collaboration systems (e.g. Figma). In all these cases back pressure is to be avoided.