incrementing the variable while the memory is read

Question

While I was reading through glibc source code, I found this interesting comment in strcat.c . Can anyone explain how does this optimization work?

/* Make S1 point before the next character, so we can increment
         it while memory is read (wins on pipelined cpus).  */
      s1 -= 2;

      do
        {
          c = *s2++;
          *++s1 = c;
        }
      while (c != '\0');

Answer 1

Pipelined CPU's can do some things in parallel. For instance, it can increment the address of S1, while reading from the address it used to point at.

Answer 2

It just means that the increment of s1 can be done while a char is being fetched from *s2, so it's free.