How does the strtok function in C work? [duplicate]

I found this sample program which explains the strtok function:

#include <stdio.h>
#include <string.h>

int main ()
{
    char str[] ="- This, a sample string.";
    char * pch;
    printf ("Splitting string \"%s\" into tokens:\n",str);
    pch = strtok (str," ,.-");
    while (pch != NULL)
    {
        printf ("%s\n",pch);
        pch = strtok (NULL, " ,.-");
    }
    return 0;
}

However, I don’t see how this is possible to work.

How is it possible that pch = strtok (NULL, " ,.-"); returns a new token. I mean, we are calling strtokwith NULL . This doesen’t make a lot sense to me.