Inside the switch statement

[Sam Hu]

To save time ,just get to my point:I do not understand why such code below inside the switch ... case statement is allowed ,what is the point of do ... while...:

This example uses a mixin to implement a generic Duff's device for an arbitrary statement (in this case, the arbitrary statement is in bold). A nested function is generated as well as a delegate literal, these can be inlined by the compiler: 
template duffs_device(alias id1, alias id2, alias s)
{
    void duff_loop()
    {
	if (id1 < id2)
	{
	    typeof(id1) n = (id2 - id1 + 7) / 8;
	    switch ((id2 - id1) % 8)
	    {
		case 0:        do {  s();
		case 7:              s();
		case 6:              s();
		case 5:              s();
		case 4:              s();
		case 3:              s();
		case 2:              s();
		case 1:              s();
				  } while (--n > 0);
	    }
	}
    }
}

void foo() { writefln("foo"); }

void test()
{
    int i = 1;
    int j = 11;

    mixin duffs_device!(i, j, delegate { foo(); } );
    duff_loop();	// executes foo() 10 times
}

Thanks in advance.

Regards,
Sam