Lets say you have 100 people and 10 seats for government and your electorate is 60% blue and 40% red. If the elections are representative, your government would be 60% blue and 40% red. But if red gets to choose who votes for who, they can have 3 seats with nothing but blue voters. That leaves 7 seats open and 70 voters, each of those seats are now voted for by the remaining 40% red and 30% blue. Red wins all those 7 seats, giving them a 7-3 supermajority instead of the 4-6 minority that proper representation would call for.