Mario Draghi, president of the European Central Bank (ECB), addresses the media following a meeting with the ECB's council in Frankfurt am Main, western Germany, on September 6, 2012. - 

Sarah Gardner: Here's the scintillating headline from a press release we got from the European Central Bank this morning: "Technical Features of Outright Monetary Transactions." Well, turns out that sleep-inducing headline is the big story today and helped inspire a surprisingly big rally on Wall Street. The S&P 500 closed at its highest level in four years.

So, what happened? The European Central Bank announced its so-called "big bazooka" -- a new plan to save the euro and stem the debt crisis. The bank's going to buy the bonds of struggling eurozone governments that ask for help. In return, more austerity.

From the European Desk in London Stephen Beard reports.

Stephen Beard: If you've ever wondered what a big bazooka sounds like here it is:

Mario Draghi: We will have a fully effective backstop to avoid destructive scenarios.

ECB chief Mario Draghi says he'll defend the euro. He'll buy up those eurozone government bonds that investors are dumping and so keep borrowing costs in check. We're talking really impressive weaponry here, says Pierre Briancon of Reuters Breaking Views.

Pierre Briancon: It is definitely from the European Central Bank's point of view a big bazooka because the key word here in their new policy of buying sovereign bonds is "unlimited."

Who can match the ECB's firepower when it can print its own money? The plan did not disappoint. Stock markets on the both sides of the pond jumped for joy. But not everyone was impressed.

Beard: Is this the big bazooka?

Raoul Ruparel: In a nutshell, no.

Raoul Ruparel of the Open Europe think-tank.

Ruparel: Ultimately it tackles the symptoms of the crisis, but not the underlying causes of the crisis.

He says Draghi's new weapon won't zap the euro's fundamental problem: the big divergence between the strong and weak economies that use the same currency. He thinks the market enthusiasm will fizzle. And within a month Mario's bazooka may look more like a BB gun.

In London, I'm Stephen Beard for Marketplace.

Want to understand the ECB's bond-buying plan? Watch a video explainer from Sr. Producer Paddy Hirsch.