MADRID (AP) — Investors and governments have applauded the European Central Bank’s ambitious plan to ease Europe’s financial crisis by buying unlimited amounts of government bonds.

The plan was devised to help out Italy and Spain, which are struggling with large debts and high borrowing costs.

After the bank’s president, Mario Draghi, unveiled the plan Thursday, stock markets across the globe have rallied. More importantly, Spain and Italy have seen their borrowing costs fall.

But the ECB’s offer of help is no gift. It requires countries that want help to first apply to the eurozone’s bailout fund. That means agreeing to tough conditions — ‘‘strict and effective’’ adherence, as the ECB puts it, to cutting down on spending, deficits and debt.

So far, the leaders of Spain and Italy — trapped between pitiless markets and voters angry over austerity — have refused to commit to the latest scheme to bring an end to a crisis that has tormented Europe for the past three years and put a drag on global economic growth.

Here are key factors involved in the rescue plan:

—What would Spain and Italy need to do to get ECB help?

Draghi’s plan involves potential purchases of unlimited amounts of a country’s short-term government bonds, a step that would lower their borrowing costs. Buying government bonds in the open market would drive up bond prices and push down their interest rate, or yield. Then governments could take advantage of those lower costs the next time they borrow. They must constantly sell new bonds to pay off old bonds as those loan repayments fall due.

However, countries wanting central bank help must first ask the 17 countries that use the euro for an assistance program from the eurozone’s existing bailout funds. The funds would have to join the ECB effort by buying bonds directly from those governments.

And they must submit their economic and budget policies to scrutiny by the International Monetary Fund.

Only then will the ECB step in — if it’s satisfied the requirements are tough enough — and buy bonds with remaining maturities of one to three years on the open market.

— How is this different from previous bailouts?

The financial rescues of Greece, Ireland and Portugal meant lending them enough money so that they didn’t have to go to international debt markets — which were charging unaffordable interest rates — to keep paying off maturing bonds.

Under Draghi’s proposed remedy, the countries would continue to raise money from private investors. The ECB would act to ease the cost of borrowing.

That could end up annoying previous bailout recipients — who remain locked into painful debt-reduction strategies in return for the help.

— Why should Spain sign up to the ECB plan?

Spain is stuck in a double-dip recession and unemployment is close to 25 percent. Spanish Prime Minister Mariano Rajoy has already accepted an international loan of up to €100 billion ($127billion) to rescue banks reeling from a collapse in real estate values. But the government is struggling to pay punishingly high interest rates to raise money on bond markets and keep its economy functioning. The interest rate — or yield — Spain’s 10-year bonds was 5.69 percent on Friday, while Germany’s was 1.52 per cent. That wide gap may force Rajoy to take up Draghi’s offer.

However, Rajoy may want to try and negotiate the terms of ECB intervention. And he has several factors in his favor. Spain has already raised three-quarters of all the money it needs so far this year. The mere announcement of the ECB plan has lowered its borrowing costs in the market. That means Spain can carry on a bit longer without outside help — while Rajoy holds out for easier conditions for an eventual rescue. His trump card: He knows his eurozone partners could not allow a catastrophic collapse of Spain’s finances which would hurt them, too.

James Hughes, chief market analyst for London’s Alpari UK, warns: ‘‘If he spends too long negotiating with the European creditors, the market will get frustrated and we'll quickly see yields move higher again.’’

Other European leaders may press Spain for an answer before an October 18-19 summit. The country will also need to borrow more money in late October to pay off bonds that are coming due.