The garter toss tradition started way back in the day when the bride and groom were forced to have witnesses(normally family&friends)in the room the wedding night to prove their marriage consummation.Those witnesses tried to get under the bride's dress to get the undergarments because they were considered good luck,thing that wasn't much appreciated by the brides and some grooms so grooms started throwing the garter that way the people in the room would not try to obtain it themselves.