Par2 checking happens as each rar is downloaded, with a final check (& repair if necessary) & unrar to destination after the queued collection is @ 0MB (not the whole queue) ... assuming you've set it up right.
You can watch the progress of checking on the PAR2 tab (expand the par2 entry) - each rar file that checks out fine will be highlighted green. There must be at least one .par2 in the download folder for this to work. If you spot any problems, check your Log tab for error messages. In fact, ALWAYS check your Log, it has all the answers.
P.S. My advice: pause those 'large items' for now and test with something smaller until you're comfortable with how it all works.