From 5W Infographics, a breakdown of how the bargain basement carriers do it:

AUTHOR
Mick Weinstein
Mick is the Head of Editorial for Covestor, a financial journalist and online content specialist. Prior to joining Covestor, Mick was for five years the Editor in Chief and VP Content at stock market analysis website Seeking Alpha, where he built the editorial function as the site attracted over 3.5 million unique monthly visitors and developed an innovative platform for intelligent stock market discussion. Mick is a graduate of the University of Michigan, Ann Arbor.
380 posts You may also like
The current bull market, up nearly 300% since 2009, has had 5 corrections
The central bank’s level of support is off the charts
While outcomes are always highly uncertain, a consistent, disciplined, and repeatable investment process is firmly in their control.
Low growth and sky-high prices may be in store for global investors