How is the Treasury Index used in relation to mortgages ?

The Treasury Index is an index used to determine interest rate changes for certain adjustable rate mortgages (ARMs). It is based on the results of auctions that the U.S. Treasury holds for its Treasury bills and securities or is derived from the U.S. Treasury's daily yield curve, which is based on the closing market bid yields on actively traded Treasury securities in the over-the-counter market.