DiskPerfDeviceControl () of diskperf in wdk;

In the function DiskPerfDeviceControl (), for the control codes that
diskperf does not process it does the following:

...
else {
//
// Set current stack back one.
//

Irp->CurrentLocation++,
Irp->Tail.Overlay.CurrentStackLocation++;
}

I do not what understand what this code is doing? I believe the intent
here is to just pass on the control codes to the next driver. So
shouldn't it be doing IoCopyCurrentIrpStackLocationToNext (), so that
next lower driver gets the same information as diskperf. Also WDK says
that undocumented members of _IRP are for IO manager only, then why
they are being used here.

Please help. Thanks.

.