During the repayment period you'll pay down what you owe by making a monthly payment calculated using the interest rate in effect at the start of your repayment period.
When rates decrease, less interest is due, so more of your monthly payment repays the principal balance.
When rates increase, more interest is due, so less of your monthly payment repays the principal balance. In this case, you may need to make a single "balloon" payment to cover your unpaid balance in full at the end of your repayment period.
For Texas HELOCS different rules apply: if the interest rate increases during your repayment period, then your monthly payment will also increase in order to repay your balance by the end of the repayment period.