Golang选择触发器

I am working on the GoLang Tutorial Select.go

func fibonacci(c, quit chan int) {
    x, y := 0, 1
    for {
        select {
        case c <- x:
            x, y = y, x+y
        case <-quit:
            fmt.Println("quit")
            return
        }
    }
}

func main() {
    c := make(chan int)
    quit := make(chan int)
    go func() {
        for i := 0; i < 10; i++ {
            fmt.Println(<-c)
        }
        quit <- 0
    }()
    fibonacci(c, quit)
}

For the 2nd case statement, I understand that it will be triggered when something comes through the quit channel. But I don't understand why the 1st case statement will be also triggered. It is just plainly pushing into pipe. It does not look like some "wait-and-trigger" scenario. More broadly, what are the criteria to judge if the case statement is triggered?

In this case it looks like some "wait-and-trigger" scenario. One can send in unbuffered channel only when receiver side of pipe can receive. Unbuffered channels are like rendezvous. So c <- x can't proceed before fmt.Println(<-c) done with print and ready for receiving new value. In case of many possible choices case is chosen via a uniform pseudo-random selection. So the criteria to judge is just a probability. If many goroutings want to send or receive from a channel, then perhaps it would be triggered rather then rare case.