Well, imagine we have a state where the number of Democrats and Republicans are equal. This state, called Rectangulus, is made entirely of rectangles. In fact, there's 30 equally sized rectangles that all contain the same number of people, in a perfect 5 by 6 array. The state has been apportioned 5 districts, which means that each district will have at 6 people, if there is population equality. If a district has 3 blue squares and 3 red squares, it's a tossup district. Any other split makes the district safe for the party with the most seats. Well, let's see what happens when we gerrymander it for each party.
See how when the maps are gerrymandered, despite the two parties having an equal number of votes, the party who is in control of gerrymandering has enough safe seats to ensure that it will be in power, even though the vote would be a tossup. It would extremely difficult for the minority party to win the seats that they need. A political gerrymander aims to increase the number of wasted votes that go towards the opposing party. The way politicians make districts favor their party is by packing voters of the opposite party together and spreading their own voters out by enough so that they do not make competitive districts but rather districts that narrowly favor their party. Wasted votes are votes that are made in a district that a party loses, or every vote greater than 50% of the voting area. The efficiency gap, the difference between the percent of wasted votes between two parties, is a measurement that is being used to determine if a map is biased. In races where the vote is tied, there are zero wasted votes. When the race is one by one vote, there are zero wasted votes for the winning party and all the votes for the losing party would be considered wasted. For example when the Democrats have a gerrymander in Map 1, they have 2 wasted votes in District 3 and 1 wasted vote in District 5, summing to 3 wasted votes. Republicans on the other hand waste 2 votes in Districts 1, 2 and 4 and 1 vote in District 5, ultimately wasting 7 votes. To get the efficiency gap, we find the difference between the two numbers of wasted votes, and we divide that by the total number of votes cast. In this scenario, we would subtract 7 by 3 to get 4 and then divide 4 by 30, to get a efficiency gap of 12%. Generally an efficiency gap of 7% is considered the borderline for gerrymandering.