Form, the first aggregate, stands for physical experiences, the body. What is called the body is actually a concept of the body, and not any real thing. If you look at your present experience, do you find the body anywhere? Since there is no body found, it is absent, it is empty. Because it is empty, there is nothing to be attached to, nothing to grasp. When you do not grasp the body, the body is dropped. Same with the other four aggregates that constitute the mind, hence mind is dropped. Body and mind dropped is zazen.