The option ARM allows borrowers to pay less interest than the formal rate for a limited period (the vast majority of customers choose this option). In return, the unpaid interest is added to the original loan, a process soothingly called “negative amortisation”. While house prices are rising, the product just about makes sense. If borrowers do get into trouble when they start paying off the loan in full, higher property values offer some wiggle-room. But when house prices are falling and refinancing is difficult, as is now the case, the option ARM is the financial equivalent of a bikini in winter. Homeowners end up owing more on a property that is worth less.