The idea behind gerrymandering is pretty simple: you pack your opponents’ supporters together into very few districts. Then you make other districts relatively more balanced — but you place enough of your supporters in most of them to give you an advantage. The hoped-for result is that your party loses a few districts hugely, yet wins a majority of districts comfortably.
North Carolina is a particularly illustrative case. To understand why its congressional delegation remained 10-3 Republican despite half the votes being cast for Democrats, look at how some of the state’s districts are drawn:
Pay particular attention to the long, snakelike districts — the 12th, in pink, and the 4th, in purple. In defiance of geographic logic, these districts cut through most of the state and encompass multiple urban centers that aren’t even close to each other. So a great deal of the state’s Democratic-voting population is packed into these two districts.In all, North Carolina Republicans created three incredibly Democratic districts, and then a larger number of districts where Republicans had a somewhat smaller advantage.
This is a very successful partisan gerrymander: even in a Democratic wave year like 2018, it held up — only one race (in the 9th district) ended up being close, and the Republican even won there.