The system must always provide feedback to the user so that they know and understand what is happening at every point in the process. Creating usable systems which match the definition is a very difficult process. The difficulties lie in the open ended usability definition and what processes work for some users may not work for others. The following is meant to illustrate the usability problem. Consider the definition given above.
Firefighters can use it wearing thick asbestos suits.
It can be used by a tribesman who has no idea what a computer is to explore the world wide web. Two words which underly HCI are consistency and feedback. A system must always be consistent. The user interface should always be the same. Switching from one style of interface to another will only seek to confuse the user.