See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question.
Question: What is I-485 Pre-adjudicate/Pre-adjudication?
You are at the last step of the green card process. Your priority dates became current, you filed the I-485, then the priority dates slipped back two years, at this time USCIS has two choices. They can either put your file in their filing cabinet and forget about the case, until the priority dates become current again. Or they can sit down and can say look we have time, let’s go over the case, adjudicate all the details, decide that the case is completely qualified to receive a green card, so we pre-adjudicate, decide that the person is entitled to get the green card. Make a note on the file. If you need some documents, we send an RFE get the documents so when the priority dates become current again all we have to do is issue the green card. That’s pre-adjudicate.
Question: How soon must I join my future green card employer?
There is really no rule of thumb. But within a commercially reasonable period of time. Three to four months maybe, after approval of the green card, you should join the future green card employer. In your particular case that appears to be a requirement sometimes it may not be a requirement because of AC21 and remember AC21 portability is available even in future employer green card cases.