This state is not permanent, and activates only when you set the CheckBox1.Checked.
To better understand, try this code:
procedure TForm1.CheckBox1Change(Sender: TObject);
begin
if CheckBox1.Checked then
SetWindowPos(Handle, HWND_BOTTOM, 0, 0, 0, 0,
SWP_NOMOVE or SWP_NOSIZE)
else
SetWindowPos(Handle, HWND_TOPMOST, 0, 0, 0, 0,
SWP_NOMOVE or SWP_NOSIZE);
end;