In Transmission, if you right-click on a paused torrent, you will get a menu. One option is "Start" and the one below that is "Start Now".
Neither of these options have any tooltips associated with them, nor is there any information on the official webpage.
start
it'll put it in the queue until a download spot opens up. If youstart now
it starts it even if all download spots are currently downloading. Just a guess, but makes sense