Tasks are usually defined to run at a specific time or as soon as possible afterwards. Hence if your computer is not on at Wednesday @1:00AM, it will run as soon as you next boot up. Background tasks do not impede a user from working.
WRT the running of it weekly, you obviously did not read the articles linked above. On the other hand, nothing prevents you from changing the schedule to what you want. The times I mentioned earlier are the default times.
However if you are stuck in thinking of the defrag methodology prior to Vista, there are additional papers on the PerfectDisk site. These papers do not only relate to defragging using PerfectDisk but defragging in general.
When I first started in computers over 40 years ago (yes before PCs existed) every job/task was a batch job. In the 40 years, I have seen jobs when I used to run a datacenter move from batch to foreground to background. I do miss the days when I would re-sequence a disk/defrag, shutting down all users, and reading a newspaper while it ran. Today I am able to post in this forum, defrag my drive and drink tea at the same time.