Cain's website says that Phase II will use The Fair Tax plan. Under that plan, the sales tax would be 23% (but apply only to services and new products being sold to end users).
The plan assumes that someone making $24,800 is spending his or her entire salary in such a way that every dollar earned is taxed. The government then sends that person quarterly or monthly checks equal to 23% of $24,800. People earning less than that receive larger checks so that everyone below that level has a negative tax rate. At $14,700, the tax rate becomes -23%. People earning more than that also receive checks - but the amount is smaller - someone earning $58,800 ends up with an effective tax rate of %11.
Those making $235,000 end up with a rate of 20% and those making $914,000 end up with a rate of 23%.
A little complicated but it does meet the definition of a progressive tax plan.