go - Slicing a sliced reference

I'm taking the tour on Golang site, and I'm trying to digest one of the examples. It is unclear how it works:

package main
import "fmt"
func main() {
    s := []int{2, 3, 5, 7, 11, 13}
    printSlice(s)
    // Slice the slice to give it zero length.
    s = s[:0]
    printSlice(s)
    // Extend its length.
    s = s[:4]
    printSlice(s)
    // Drop its first two values.
    s = s[2:]
    printSlice(s)
}
func printSlice(s []int) {
    fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}

The output is:

len=6 cap=6 [2 3 5 7 11 13]
len=0 cap=6 []
len=4 cap=6 [2 3 5 7]
len=2 cap=4 [5 7]

After the first slice, s = s[:0] the slice length is 0. Then there is another slicing of s = s[:4]. Although the length is 0, this seems to work. But how this happens? Shouldn't the underlaying array be in accessible from s?

What confuses me more is, the next time we slice it, s = s[2:] we slice the old value of s (which is 4 elements) and not the original array.

Can someone shed some lights what is the difference between the two cases?