You have to apply for the loan repayment. If you get accepted, the years are added on to the end of your HPSP commitment.
The loan repayment $ amount is going up. In my opinion it is a good deal if you are going to do an AEGD and work as a general dentist. BE CAREFUL if you want to specialize. The loan repayment years are served consecutively NOT concurrently. For example, you do a two year specialty residency, you owe two back. THEN you have two more left for the loan repayment.
If you plan on staying in for a career, you can make more money with a multi-year retention bonus than a loan repayment.
If you do your HPSP time then two or three years for loan repayment and that's it (4-6 years) it could be a good deal. Ask a lot of questions and READ the contract.