It doesn't. You are not required to notify BCIS that you have invoked AC21. Some lawyers do recommend that you notify them that you have joined a new employer. Here is a quote from murthy.com:
AC21 does not state that notification to INS is required. The current INS guidance on the issue says that it is "expected" that a letter will be sent to INS. There is also no particular format for sending a letter advising of the change in jobs.
Some lawyers do advice that you notify BCIS, but it is not required.
rgds,
sadiq
