The trump card the ECB could use on Greece

Greece's economy is living on borrowed time—particularly if you believe the old adage that time is money.

That's because 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). Otherwise known as ELA, emergency liquidity assistance is a loan program available for national banks in distress, allowing access to cash at an interest rate set by the ECB's Governing Council.

"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."

Read More Greece defiant; accused of 'breaking all the rules'

All of the risk falls on Greece if the loan can't be repaid, unlike normal circumstances when a country has adequate collateral to offer and can receive a loan from the ECB at a lower interest rate.

"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.

But since the ECB rejected Greek bonds as collateral, ELA has become the last resort for the troubled nation that's seeing deposit outflows in commercial banks peak at 300 million euros a day.

Read MoreGreece on 'brink of disaster;' calls emergency meeting

Without ELA propping up Greece's commercial banking system, banks would be called to repay the National Bank of Greece. Without enough cash to do just that, banks could close, deposits could be lost, depression could ensue. It's powerful leverage the ECB holds.

"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."

From all indications, however, the ECB remains dedicated to keeping Greece in the euro zone, and a risky move like that could lead to unforeseen consequences that carry the potential to backfire. But it wouldn't be the first time the possibility has arisen.

Read MoreAs clock ticks, Greece risks running out of friends

When Ireland had become increasingly reliant on ELA—peaking at 187 billion euros in 2011—the ECB reportedly used the notion of cutting the emergency funds if Dublin didn't agree to bailout terms, which the country later did.

In 2013 Cyprus turned to emergency liquidity assistance after the ECB, similarly to the Greece case, refused to accept the country's sovereign bonds as collateral. Cyprus too faced warnings that the ECB's Governing Council was prepared to reject the continuation of ELA. The country later entered a rescue program and imposed capital controls to restore solvency to its banks.

Greece has until the end of the month before its next debt payment is due.

—Reuters contributed to this report.