объясните последовательность

#include<Windows.h>
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(HINSTANCE hInst,HINSTANCE prev, LPSTR cmdline,int ncmdshow)
{
    HWND hwnd;
    MSG msg;
    WNDCLASSEX wc;
    char* ClassName="ClassName";
    char* MenuName="MenuName";
    wc.cbSize=sizeof(wc);
    wc.style            = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc      = WndProc;
    wc.cbClsExtra       = 0;
    wc.cbWndExtra       = 0;
    wc.hInstance        = hInst;
    wc.hIcon            = LoadIcon(hInst,IDI_APPLICATION);
    wc.hCursor          = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground    = (HBRUSH)(COLOR_WINDOW+1);
    wc.lpszMenuName     = MenuName;
    wc.lpszClassName    = ClassName;
    wc.hIconSm          = LoadIcon(hInst,IDI_APPLICATION);
    RegisterClassEx(&wc);
    hwnd = CreateWindow(ClassName, MenuName, WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, NULL, hInst, NULL);
    ShowWindow(hwnd,ncmdshow);
    UpdateWindow(hwnd);
 
    while(GetMessage(&msg,NULL,0,0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT imsg, WPARAM wParam, LPARAM lParam)
{
    HDC hdc, mdc;
    static HBITMAP* hbm;
    static int x, y;
    switch(imsg)
    {
    case WM_CREATE:
        hbm = new HBITMAP;
        SetTimer(hwnd, 1, 100, NULL);
        break;
    case WM_SIZE:
        hdc = GetDC(hwnd);
        mdc = CreateCompatibleDC(hdc);
        *hbm = CreateCompatibleBitmap(hdc, 20, 20);
        ReleaseDC(hwnd, hdc);
        SelectObject(mdc, *hbm);
        Rectangle(mdc, -1, -1, 21, 21);
        Ellipse(mdc, 1, 1, 20, 20);
        DeleteDC(mdc);
        hbm++;
        hdc = GetDC(hwnd);
        mdc = CreateCompatibleDC(hdc);
        *hbm = CreateCompatibleBitmap(hdc, 20, 20);
        ReleaseDC(hwnd, hdc);
        SelectObject(mdc, *hbm);
        Rectangle(mdc, -1, -1, 21, 21);
        Ellipse(mdc, 1, 1, 20, 20);
        DeleteDC(mdc);
        hbm--;
        break;
    case WM_TIMER:
        hdc = GetDC(hwnd);
        mdc = CreateCompatibleDC(hdc);
        SelectObject(mdc, *hbm);
        BitBlt(hdc, x, y, x + 20, y + 20, mdc, 0, 0, SRCCOPY);
        ReleaseDC(hwnd, hdc);
        DeleteDC(mdc);
        hbm++;
        hdc = GetDC(hwnd);
        mdc = CreateCompatibleDC(hdc);
        SelectObject(mdc, *hbm);
        BitBlt(hdc, x + 40, y, x + 60, y + 20, mdc, 0, 0, SRCCOPY);
        ReleaseDC(hwnd, hdc);
        DeleteDC(mdc);
        hbm--;
        x++, y++;
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hwnd,imsg,wParam,lParam);
        break;
    }
}