Potentially unlimited intervention by the European Central Bank(explain this) in the bond markets, which could be announced at the ECB’s September meeting, is the latest to catch the attention of the markets (click here for more on the market reaction). If enacted — and don’t forget the ECB has denied reports that it may do this — this could be the “ultimate game-changer in the euro zone debt crisis,” according to Stephen Major, strategist at HSBC .

“Not only would it drive spreads down, it would keep them down,” he argued.

A report in respected German magazine Der Spiegel that the ECB is considering setting a target for bond yield spreads in southern Europe buoyed stock markets and sent Spanish and Italian bond yields down earlier this week. The gains were reversed after an ECB spokesman called the report "misleading." (Read More: Germany Denies Knowledge of ECB Bond-Buying Plans.)

Of course, there have been plenty of possible “big bazookas” mentioned before — a euro zone banking union and the ECB’s injection of liquidity into the banking system via long-term refinancing operations. Major cautions that even if this latest potential intervention happens, political action is still needed. (Read More: What Is an LTRO Anyway?)

“Harnessing the ECB’s lender-of-last-resort firepower, more of which will be revealed by 6 September, to the significant muscle of the (European Stability Mechanism), expected to be operational after 12 September, provides a big incentive for governments to continue reforms. But governments still have to deliver,” he wrote. “While the ECB cannot on its own revive European economic growth, it can help troubled countries refinance themselves, which would solve one important element in this crisis.”

Soaring bond yields are one of the factors which have driven euro zone peripheral economies into the hands of the troika of the International Monetary Fund (explain this), ECB, and European Commission. Spain and Italy are the latest countries whose rising cost of borrowing indicates they may need a bailout.

After a quiet August, events in the euro zone are likely to become a headache for markets again in coming days, as recently elected Greek Prime Minister Antonis Samaras starts a series of meetings with euro zone leaders. He is expected to ask for a two-year extension to Greece’s bailout terms, which are extremely unpopular within the country. (Read More: Euro Zone Chief Arrives in Athens for Tough Talk.)