This is an understanding that comes from habitual practice. Only wisdom can pay attention to the mind door. If you understand the mind, then you can continue to be aware at the mind door.
Because the mind is not at any place – the mind is nowhere, no shape, no place – therefore, you don’t have to focus, but you can know. If you know many times that this is happening, this is happening, this is happening, then slowly the mind understands how to observe the mind as an object.
Now you know that the mind is thinking, right? That is the mind door. Do you know your intention to do something? This is also the mind. If you know the mind, that is the mind door.