C Language question

Hello,
I was working on a ARM project with my Keil toolchain, and by chance I just found that the following code is working (see case 2); I tested it in the ARM simulator using different values for alfa and beta.
Someone knows where can I find more detailed explanation about this behaviour of C language (K&R say nothing about "case" into "if"...)?
Thanks in advance.
Bruno

char alfa;
char beta;
char zeta;

int main (void)
{
while (1)
   {
   switch (alfa)
      {
      case 0:
         zeta = 17;
         break;

      case 1:
         zeta = 91;
         break;

      case 2:
         if (beta == 1)
            {

            case 3:
               zeta = 38;
               break;

            case 4:
               zeta = 47;
            }
         break;
      }
   }
}