Golang指向切片和数组的指针(Golang pointer to slice and array)

I was looking at golang's heap package's (https://golang.org/pkg/container/heap/) Priority Queue example and came across this:

type PriorityQueue []*Item
...
func (pq *PriorityQueue) Pop() interface{} {
    old := *pq
    n := len(old)
    item := old[n-1]
    item.index = -1 // for safety
    *pq = old[0 : n-1]
    return item
} 

When I started playing around with this code to make sure I understood it, I tried:

item := *pq[0] // error

This gives you type *[]T does not support indexing. But if you do:

item := (*pq)[0] // all is well

This is type assertion right? Was hoping someone could explain what is going on here.

Here is some code to quickly show this: https://play.golang.org/p/uAzYASrm_Q