These are threats to give bad reviews or spread unflattering information (whether true or not), if the threatening negotiator does not get their way.
“If you don’t give me what I want, I will trash your firm on Yelp.”
You can usually take steps on the front side of a transaction to ensure that this tactic is never threatened. Build up good will before the transaction. Sign a non-disclosure or confidentiality agreement at the outset. If it is too late, you can always comment. Talk to a social media optimization professional. Take legal action in the case of libel or violation of privacy laws or of a contract.