Strategic preparation is a cornerstone of negotiation success. While we must also adeptly deal with tactics, if we do not take care of strategic concerns, tactics alone are unlikely to optimize a negotiation outcome. Simply showing up and declaring you are not prepared is not a legitimate tactic, and if it happens, you can neither be forced or force anyone else to negotiate. Preparation is implied among negotiators, and well-prepared negotiators generally prevail. It is very difficult to defeat a completely prepared and operationally and tactically competent negotiator. Being unprepared is a poor situation even when used as a stall tactic. This may be legitimate for some extreme excuse, but you can retreat if new substantive information or circumstances are exposed at the table anew. Then you can say you are not prepared to address that point.
A common example is, “I am not prepared to consider that, as that is new information.” This may or may not require consideration.
If this tactic is used earnestly, you must negotiate about its relevance in delaying the negotiation. Focus on the new information. If the “what if” scenarios are unknown or complex, a break may be warranted with specific times to decide on the issue a negotiator was not prepared to discuss.