C Puzzle #13

void duff(register char *to, register char *from, register int count)

  {

      register int n=(count+7)/8;

      switch(count%8){

      case 0: do{ *to++ = *from++;

      case 7:  *to++ = *from++;

      case 6: *to++ = *from++;

      case 5: *to++ = *from++;

      case 4: *to++ = *from++;

      case 3: *to++ = *from++;

      case 2: *to++ = *from++;

      case 1: *to++ = *from++;

              }while( --n >0);

      }

  }

Is the above valid C code? If so, what is it trying to acheive and why would anyone do something like the above?

Answer is here