I don't get this. Can you elaborate? So you're saying because I bought an iTunes gift card that was $100 for $80, Apple will only ever get $10 of my actual doll

I don't get this. Can you elaborate? So you're saying because I bought an iTunes gift card that was $100 for $80, Apple will only ever get $10 of my actual dollars if I spend all $100? This is an interesting hack.