In short, the advantages of the two-round system hinge on the consistency with which interparty negotiations between rounds produce stable coalitions. If they do not, two-round elections can produce perverse results. Despite its name, the system that he envisions would be essentially parliamentary, with the presidential option as a back-up. Both an assembly and a president would be elected in concurrent, two-round elections, but normally the presidency would be completely dormant. The president's purpose would be to encourage what Sartori considers to be responsible behavior on the part of the government and parliament.
Responsibility is measured by executive stability.
Sartori is not shy about investing authority in his back-up executive. He suggests endowing the president with the authority to make law by decree in inverse proportion to his support in the assembly. The rationale for this counterintuitive setup is that the more severe the repercussions of bringing down a government under the parliamentary mode, the greater the incentive for parties to be internally disciplined and to form stable governing coalitions. The president is an 800-pound gorilla whose mere existence is supposed to persuade legislators not to wander into presidentialism. Scholars of comparative constitutions, including Guillermo O'Donnell, Adam Przeworski, Scott Mainwaring, and Matthew Shugart, have offered abundant evidence, both theoretical and empirical, that dominant presidents are problematic for democratic performance.