This occurs when time has been invested in a relationship or deal and there is a strategic concern, impasse, or blockage to the trajectory of the negotiation. An offer to be more transparent will be followed by a request for concession. This can break an impasse and create trust, but it can also blow your leverage if you disclose your walk-away point. According to studies, truth is better with reasonable and/or ethically non-compromised people[v].
Consider a negotiator saying, “We’ve been arguing over the costs and viability of this deal. If we show you our itemized budget, can you show us your cost of manufacture?”
The way to safely gain the benefits from this tactic, is to share information pursuant to a confidentiality or non-disclosure agreement. It is best if these agreements are in place at the start, where they may then be used or augmented to accomplish your goals without giving away your leverage.