Some ARM loans have payment caps, meaning your monthly payment cannot increase above a certain amount, even if the interest rate rises sharply.
The danger is something called negative amortization.
Suppose your payment cap is $500 per month, but your interest rate increase should have resulted in a payment of $550 per month. Your lender does not just forgive the extra $50 in interest. No, your lender takes that $50 per month out of your equity.
Rather than amortizing your loan so the principal balance gets lower each month, you are negatively amortizing and your principal balance is getting higher every month.