‘Sortition’ means ‘choice by lot; random selection’.
Random selection in a large population will automatically produce, with small deviations, a proportional representation of that population. This is ‘automatic’ because of probability theorem: ‘The Law of Large Numbers’.
A random selection of, say, 1,000 people from the U.S. population would produce a body that would contain approximately 492-496 men and 504-508 women (since men are 49.4% and women are 50.6% of the national population).
Unlike selection by competitive balloting, sortition is immune to the sway of political faction, economic privilege or any other factor besides citizenship.
[Note, however, that our proposal also suggests one other qualifier: a base-level understanding of the legislative process.]