According to this article about Bridgegate:
Then, in a dramatic twist, Brennan called for any New Jersey citizen currently serving on a grand jury to exercise their legal right to summon him to testify before them and indict the governor.
"If anybody in the state of New Jersey is currently sitting on a grand jury, I implore you: Summon me before you," urged Brennan. "I will come before you with the transcripts and the evidence and you can get an indictment. Any grand jury, without prodding from the prosecutor, can call witnesses and demand evidence and demand this case be prosecuted...without the prosecutor's help."
Dennis Kearney, a former assistant prosecutor in Essex County who's now a partner in the criminal defense firm of Day Pitney, said such a move, while technically possible, would be unprecedented, and "very unlikely" to happen.
"The idea that a grand jury would 'go rogue'?" asked Kearney. "I've never seen it, and I go before grand juries for a living."
The phrase "demand this case be prosecuted" is rather ambiguous, and I'm not sure whether they mean that the jury could air Christie's dirty laundry in public until the prosecutor agreed to formally ask for an indictment, or that the jury could actually perform the indictment on its own.
Do grand juries have the power to indict people all by themselves, or does the prosecutor have to initiate the process? Are there any known examples of a grand jury "going rogue" as they describe?