Alt1 reads the xpcounters from RuneMetrics. The app will keep track of the changes in these counters to calculate your xp rate. You can either choose a fixed interval, or set a start time and calculate the xp/hr from that point on.
You need to have the RuneMetrics xp counters enabled in order for this app to work. You can open these by hovering over the RuneMetrics icon on the ribbon interface and clicking Metrics in the sub menu.
You can also use the xp meter to keep track of how often you have done a certain action. To do this, click on this icon . You will now see a list of different xp drops that were detected and how often they happened. You can click the eye button to track the amount of time that a specific xp drop happened. This way you can keep track of your slayer tasks real time.
You can use this slider to choose after how much time Alt1 should warn you that you are no longer getting xp. When the selected amount of time passes without gaining xp, you will be notified with a tooltip by your cursor.