Excel UI thread freezes on async function invocation

I am kind of confused, I was following the "Excel 2013 SDK" documentation, and bein interested in User defined Asynchronous functions. Here is my simplest example, it doesn't even do anything:

// registration
    { L"Func5",                                 // Procedure
        L">QX",                                 // void return >; 
        L"Func5",                               // function_text; The function name as it will appear in the Function Wizard
        L"Arg",                                 // argument_text; The user sees this in the Function 
        L"1",                                   // macro_type; "1" can be called from anywhere; listed everywhere
        L"Generic Add-In",                      // category
        L"",                                    // shortcut_text; A one-character, case-sensitive string that specifies the control key assigned to this 
        L"",                                    // help_topic
        L"Sig: >QX",   // function_help
        L"Argument ignored"                     // argument_help1
    },

// ...

__declspec(dllexport) void WINAPI Func5(LPXLOPER12 x, LPXLOPER12 asyncHandle)
{
    switch (x->xltype)
    {
    case xltypeNum:
    {
        // just break, not doing anything
        break;
    }
    default: break;
    }

    return;
}

This compiles fines and Excel loads it correctly, but as soon as I type "=func5()" the whole UI freezes, and waits for me to do anything; then shows "#Calc!".

I am fine with the error, but what's even the point of Async functions if the UI freezes? Am I doing something wrong?