He used his 5% cash back credit card to buy supermarket gift cards. He gets 5% back on all gift cards he purchased (from the credit card company). He then bought money orders from the supermarket with the gift cards. There is a small fee involved say 1%. He used the money orders to pay back the credit card. He made net 4% on the total money spent for the month and his credit card balance was paid off.
In summary, just by rotating the money through: credit card buys gift card, gift card buys money order, money order pays back credit card, gain of about 4% on the credit limit of the card (assuming you had the where with all to purchase that many gift cards and money orders).
Why aren’t more people doing that? Sounds like a money maker.