In SE-0314, AsyncStream, why does the unfolding function present backpressure?

[thai-d-v]

Can anybody here help me understand what it means by presenting backpressure here?

Alternatively if a source is just an async function (one that represents a backpressure) an AsyncStream can be constructed by unfolding a producing function and a cancellation handler...

struct AsyncStream<Element> {
  init(
    unfolding produce: @escaping () async -> Element?,
    onCancel: (() -> Void)? = nil
  )
  ...
}