The H-1B portability rule is stated in American Competitiveness in the Twenty-First Century Act of 2000 (AC-21), an H-1B holder can change the employer and start to work for a new employer on the date that the USCIS receives the H-1B transfer petition submitted by the new employer on behalf of the H-1B holder. 

In order to qualify for the H-1B portability, an H-1B holder must be an individual: 1) who has been lawfully admitted into the U.S.; 2) on whose behalf an employer has filed a petition for new employment before the date of previous H-1B status expiration; and 3) who has not been employed without authorization previously in U.S. 

The risk in using H-1B visa portability is that the new H-1B application could be denied. This is especially true in a climate where the USCIS is scrutinizing H-1B applications closely, as well as changing their standards and expectations as to the proof needed to meet the legal requirements for H-1B applications. 

If you rely upon a pending H-1B application to stay in the United States and work, the denial of the H-1B petition normally means that you will be out of status. This will create significant problems for you. The best way to avoid the risk of falling out of status due to reliance on H-1B portability is to avoid using the H-1B portability. Therefore, you should go from approval to approval, and not depend solely on the pending H-1B application. This is often possible when using the premium processing service to expedite the processing of the new employer's H-1B application.