How to understand the kth receive on a channel with capacity C happens before the k+Cth send from that channel completes？

Question

It comes from Channel communication.

What I really can't understand is why kth receive happens before the k+Cth send? Why not kth send or k+1 th send?

Answer 1

A capacity of a buffered channel is a number of nonblocked sends to the channel that can happen. When you have used all the capacity (C + k states about it) the next send will be blocked until a receive from the channel and only then it will complete.

If rephrase the sentence from the link with some numbers like C = 3 and k = 1 (for clarity), it will be like:
"The fourth send to a channel with a capacity 3 will complete after the first receive from the channel".