The H1 ties you to a single employer. The EAD allows you to work for another, e.g., one or more different employers, on either a full- or parttime basis. If you set up your own company and do any work for it, the company becomes another employer. The implication here is that using the EAD to work for anyone other than the employer sponsoring your H1 will invalidate your H1 status by the nature of the definition of the H1.