Can you give an example of how the old rule worked, prior to MAGI?

Before MAGI, if a state's income limit was 100% of the FPL--the state would first look at the person's gross income, then subtract out (for example) 30% of their earned income and an amount they spend on childcare as work-related expense deductions and then compare that net income to 100% of the FPL. This means that under the pre-MAGI rules, in a state with an income eligibility limit of 100% of the FPL, a person with income over 100% of the FPL can qualify for Medicaid (because of the deductions and disregards).