You're referring to the REPAYE interest subsidy which is in addition to the loan forgiveness. For example, if your monthly payment is $1000, and your interest charge for that year is $3000, you will get $1000 (monthly payment) + a $1000 subsidy (50% of $3,000-$1,000) subtracted from your loan total. PAYE and IBR do not have this subsidy.
With one of four income-driven repayment (IDR) plans, your monthly payment is based on how much you make (10-20% of discretionary income, recalculated every year based on salary, family size, etc.). After making 20 years of payments (10-25 years depending on plan), the remaining balance is forgiven (though you owe taxes on the forgiven amount). The loan forgiveness amount (less the tax on it) may outweigh the interest you'd save by paying off sooner. This is generally true if your debt is high enough (debt-income ratio ~1.5:1) even for specialists. You also have to take into account the value of the extra money you'd have instead of making loan payments.