The trump card the ECB could use on Greece

Greek banks have for months been relying heavily on what is called "emergency liquidity assistance" from the European Central Bank for just more than 80 billion euros ($90 billion).

"It's a bit like printing euros for that one national bank," said Donald Luskin, chief investment officer at TrendMacro, who points out the loan comes at a higher interest rate since it's often backed by the flimsiest of notes. "But ELA doesn't come as an obligation or a risk of the Eurosystem."

"It's strange because normally the banks that take on the risk make the decisions, but in this case it's Greece with the risk, and the ECB making the decision," Luskin told CNBC.

"This is the ultimate pressure point the ECB has on Greece, it's the real thing," Luskin said. "If the ECB says no more ELA, then Greece goes back to the Stone Age."