Only the two leading candidates from the first round go forward. This has a number of advantages - the person ultimately elected has to get 50% of the votes cast (a disappearing quality for British MPs - who can be elected on a small proportion - and the more candidates who stand, the smaller the winning proportion necessary). Representation is based on which candidate can attract the widest support (Proportional Systems can hand seats to candidates who only appeal to a particular group - and can discourage reaching out beyond the particular ethnic or extremist group). It is more expensive than expressing second (or more) preferences on a single ballot sheet (as in the Alternative Vote) - but allows the voter to choose between the most popular candidates (or parties) - without promoting the "least offensive to all" candidate.
