Планировщик go. Как происходит переключение между горутинами?

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package main

import "fmt"

func square(c chan int) {
    fmt.Println("[square] reading")
    num := <-c
    fmt.Println("[square] before writing")
    c <- num * num
    fmt.Println("[square] after writing")
}

func cube(c chan int) {
    fmt.Println("[cube] reading")
    num := <-c
    fmt.Println("[cube] before writing")
    c <- num * num * num
    fmt.Println("[cube] after writing")
}

func main() {
    fmt.Println("[main] main() started")

    squareChan := make(chan int)
    cubeChan := make(chan int)

    go square(squareChan)
    go cube(cubeChan)

    testNum := 3
    fmt.Println("[main] sent testNum to squareChan")

    squareChan <- testNum

    fmt.Println("[main] resuming")
    fmt.Println("[main] sent testNum to cubeChan")

    cubeChan <- testNum

    fmt.Println("[main] resuming")
    fmt.Println("[main] reading from channels")

    squareVal, cubeVal := <-squareChan, <-cubeChan
    sum := squareVal + cubeVal

    fmt.Println("[main] sum of square and cube of", testNum, " is", sum)
    fmt.Println("[main] main() stopped")
}

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[main] main() started
[main] sent testNum to squareChan
[cube] reading
[square] reading
[square] before writing
[main] resuming
[main] sent testNum to cubeChan
[main] resuming
[main] reading from channels
[square] after writing
[cube] before writing
[cube] after writing
[main] sum of square and cube of 3  is 36
[main] main() stopped