Sometimes you see Idle Time indicated by the grey bar under the time slots. You can actually view those idle time slots as they are recorded. The followings are the steps.
1. Go to Track Time page.
2. If you have Idle Time for the date selected, you can actually show the idle time by clicking on Actions drop-down then select Show Inactive Slots.
3. The screen shots of the idle time is actually recorded, so is the computer activity. You can view them by clicking on the icon to the left of the thumbnail images.
4. The detailed activity dialog will show that there is no mouse/keyboard activity recorded for all the 10 minutes in the interval
(If there no mouse/keyboard activity during a 10-minute interval, the interval is considered to be idle and not accounted as work time).
Here is an animated illustration
Click to enlarge and view in separate window