You enter when you finish a task (or switch to another) in the interface. The size of the block displays the relative time spent and colors correspond to weekdays.
You can view reports of the hours spent for specific timespans in the past. Which then help you keep track of what you have done for how long. For instance for reporting what you have worked on for you employer.
This is a not completely functional model for user testing. I could have gone for just a static html site to click through, but I actually wan't to get it fully funcional in the future (have not gotten around to it yet though). The design was done under huge time pressure. One of the gimmicks I envisage is a running clock displaying the correct time, this should be feasable since it is SVG.