Sometimes you download a big torrent which contains a complete TV show or several seasons and you might want to download the episodes in sequential order so you can watch them as they go downloading, there is an option to do that for you very easily

having the pack torrent selected/highlighted go to the files tab then highlight all files(Ctrl + A), then right click anywhere, then when the menu appear click on 'Prioritize by file order' this will make it download the first pieces of the torrent first in a pack this means downloading early TV show episodes/seasons first so you can watch them while other later episodes download in the background, check the image below:

http://i.imgur.com/gMURfjT.jpg

I think in other clients you will have to do it manually by using file prioritizing options

Also please note that its not recommended to use this on torrents with a small swarm because it might harm the torrent due to the scarcity of different pieces if many people do it and someone tried to download normally but didn't care about order his client will request pieces that are random and might have a very low availability rate for the later pieces which might slow the torrent down, and clients which don't have the first pieces will be ignored by the clients that only downloads first pieces that means they can't upload to them, just wanted to point that out but most of the time its fine to do that on large torrents with great health and many seeders.

Take care and happy torrenting