One reason cited [1] for why Syriza will be able to talk tough with the Troika, presuming it wins today, and can form a government, is that it has a healthy [circa 5%] primary budget surplus. That's the difference between revenues and spending, once we ignore the cost of servicing debt. The hypothesised threat is that the new Greek government renounces the debt and has no more need to borrow from capital markets, taking more in taxes than it spends.