Golang旅游＃18继续

This may be a simple thing I can't see for some reason but I am going through the golang tour and in the "For continued" section and I was wondering if someone could explain to me how the logic executes and delivers the statement to be 1024. https://tour.golang.org/flowcontrol/2

package main

import "fmt"

func main() {
    sum := 1
    for ; sum < 1000; {
        sum += sum
    }
    fmt.Println(sum)
}

Its really just doing powers of 2

so the outputs are

1 2 4 8 16 32 ... 1024

Think of this loop as

sum = 1
while(sum < x) {
    sum = sum * 2
}