They go up and down with interest rates, based on several esoteric money market indices that cause the cost of funds for lenders to vary. The most popular indices include Treasury Securities (T-Bills), Cost of Funds (COFI), Certificates of Deposit (CDs), and the Libor, which is the London inter-bank offering rate.
However, the interest rate and payment adjustments do not always coincide. There is usually a lag between the two.
A number of consumer protections have been built into these loans to keep them from fluctuating too wildly. But consumers will have to be cautious when reviewing advertising and other claims about ARMs made by lenders.