Example: earn $100k, $35k goes to taxes, spend $50k a year = $15k discretionary. Increase income 50% to $150k, taxes become $52.5k, spend the same $50k = $47.5k discretionary. So total income only increased by 1.5, but discretionary increased by a factor of 3. Of course, this is a simplistic example -- tax rates aren't constant for one thing. But the principle is very powerful, and the effect strength grows as the income increases. Using the above numbers, 2x the initial salary results in a 5x increase in discretionary income.

Lastly, this strategy also provides great protection against a decrease in income.