If your interest rate increases so that your payment is less than the accrued interest during the payment period, then your payment must increase to cover the interest. The triggering rate is the interest rate at which this will happen.
2. As you pay down your mortgage, the triggering rate will increase which makes it less likely that a payment increase will happen
3. Beyond this interest rate the outstanding amount would begin to increase. This would extend the amortization period of your mortgage.