European Central Bank President Mario Draghi gets another chance Thursday to spell out how the bank intends to rescue the 17 countries that use the euro from financial disaster.

Expectations have been high since late July when the ECB head vowed to do "whatever it takes" to hold the euro zone together.

The following week, on August 2, Mr Draghi announced the broad outlines of a plan to buy short-term government bonds to help out euro zone countries struggling to manage their debt.

Until then, countries such as Spain and Italy had seen their borrowing costs - reflected in the interest rates on bonds they sell - rise to unmanageable levels. Investors were worried the two countries could soon get to a point where they could not afford to handle their finances and be pushed into asking for a bailout.

That has already happened three times in the euro zone - with Greece, Ireland and Portugal. The worry is that Spain and Italy are too big to bail out. If those countries fail to pay their debts on time, it could spark a financial crisis that could see the euro zone break up, spreading turmoil throughout the global economy.

Analysts say Mr Draghi's comments Thursday could be constrained by the fact that the ECB may not have worked out every detail of its plans. The ECB chief has to strike a delicate balance: promise and reveal enough to keep markets happy, while making clear that further aid only will come if Europe's politicians agree to more.

Here is a look at what Mr Draghi and the ECB have been working on and what to look out for on Thursday:

By buying bonds on the open market - Mr Draghi has said the ECB will target short bonds such as those with maturities of one, two or three years - the ECB can drive up the prices for a country's bonds. That brings down their interest rate - or yield - and makes it less expensive for countries to borrow money. The ECB theoretically has no limit on the money it can use for its bond-buying plan. As a central bank, it can "print money" to pay for the bonds by simply adding to banks' reserve accounts.

How much it spends on bonds sends a message to the markets. Too much and it could be criticised for violating the EU treaty provision that forbids it from financing governments directly. Too little and investors think that ECB is only half-heartedly attempting to solve the euro zone's problems. A previous bond-buying program started in May 2010 piled up over €210 billion but was too limited to decisively lower yields.

Sticking with short term bonds is more in line with central bank's normal interest rate policy, which mainly affects short term rates, and so less open to criticism as a possible violation of the ban on financing governments. It also makes the purchases less risky, as the bonds will be paid back more quickly.

Analysts think the ECB will avoid ECB an option suggested by some: setting a firm interest rate ceiling. Once a country's yields hit a certain point, it would step in and buy the bonds. That would send a clear signal to markets but would also tie the bank's hands, forcing it to defend that red line or lose credibility.

The mere announcement last month that the ECB might intervene has already sent the yields down for Spain and Italy. Spain's yield on its 10-year bonds, for example, has fallen from a high of 7.54% in July to 6.51% this week. This is a move in the right direction but still much higher than the more financially secure euro zone countries, such as Germany, which has a 1.4% yield.

Anticipation grew more intense when Mr Draghi skipped a key central bank conference in Jackson Hole, Wyoming, because of the workload ahead of Thursday's meeting.

Potential Pitfalls

Mr Draghi has to tread a fine line - not only does he have to keep the markets from panicking and pushing bond yields even higher, but he also has to play a political game. First he has to make sure that the ECB's 23-member governing council is on his side. The bond plan still needs to be passed by at the bank's policy-setting meeting, with a majority required but a broad consensus preferred in practice. One voice critical of the bond plan belongs to Germany's central Bundesbank - an influential member of the council because of the size of Germany's financial commitment to the euro zone. Its head, Jens Weidmann, has warned that governments could become addicted to the help and avoid making unpopular spending cuts and tax increases to get their debts under control.

Mr Draghi will also have to keep governments in line if the bond-buying program is to succeed. When he announced his plan last month, Mr Draghi stressed that governments who want help must agree to certain conditions including approaching Europe's emergency bailout fund for assistance and sticking to stiff deficit rules. But once the ECB has stepped in, will countries stick to their commitments after the pressure has been taken off their finances? The history of the euro zone crisis is littered with broken promises and missed targets.

One weapon in the ECB's arsenal could be halting the bond purchases, turning up the heat by letting borrowing costs rise again. This is a risky option. It would send a strong message to the countries being helped out but it could reignite market panic and send yields spiralling.

What to expect tomorrow

The only thing that is certain about Thursday is that analysts and politicians will pore over every word uttered by Mr Draghi.

Rather than giving detailed plans about how the ECB is ready to start bond-buying Thursday, Mr Draghi is more likely give politicians another stern warning that they will have to abide by strict conditions if they want help. The pressure will be put back on leaders such as Spain's Mariano Rajoy and Italy's Premier Mario Monti to act first. Only after governments have applied to the eurozone's emergency bailout funds for help will the ECB get going with its bond buying.

The ECB could also try to stimulate more borrowing across Europe by further lowering its benchmark interest rate.

Some economists say the ECB's may set a new record low of 0.5% might be on the cards.