Negotiation 101-in what way would we NOT gain by walking out in Switzerland today?

Nevermind what our goals might be, just TECHNIQUE
In a previous professional life I negotiated with BOTH administrative units, and unions.
Especially given the Iranians balking at the west’s ‘leak’ that all U-235 was to leave Iran for Russia, this is the moment to elicit a TELL by simply saying ‘well, we tried, but I guess now the Senate will vote, next week.’ and LEAVING THE NEGOTIATIONS. If they want ANY deal at all, they call FAST. But I think their desire was to drag these negotiations out as long as they could so they had freedom to do what they need to do.
WFB:

Iran Wants More Nuke Concessions as Talks Hit Critical Stage
LAUSANNE, Switzerland—Iranian negotiators are becoming rigid and unmoving in their stance on a range of key nuclear issues in talks, according to multiple sources familiar with the negotiations who said Tehran is angling to elicit as many concessions as possible from the United States as the talks reach a critical stage ahead of the looming March 31 deadline.
Iran is pushing for major relief from economic sanctions and the ability to continue sensitive research and development on the nuclear and weapons fronts, according to sources quoted in the Iranian state-controlled press on Sunday.
The Iranian side is said to be digging in its heels over these issues as U.S. diplomats rush to finalize a tentative agreement ahead of a self-imposed March 31 deadline.
“Sanctions and research and development are the areas where Iran and the G5+1 still have differences,” one source close to the Iranian diplomatic team was quoted as telling Fars News on Sunday.
Western sources familiar with the U.S. stance confirmed the sticking points and told the Washington Free Beacon that the pressure from Iran is likely to force the Americans to offer more concessions on these fronts than have previously been on the table.
“The Iranians are again outplaying the Americans,” said one source in Europe familiar with the negotiations. “They know they’ll have to give up certain things eventually. So they’re digging in their heels on issues that mean everything and preparing to give ground on relatively minor issues—but not yet, and not until they see how much more the Americans are willing to give.”
Previous concessions by the United States appear to have motivated Iran to push harder on issues such as the possible military dimensions of its nuclear program, as well as continued work on advanced centrifuges and increased sanctions relief, according to a second American source apprised of the demands being set forth.