I’m curious why you want this, as its far better to select when you want to start and stop a download.
If you were to let a torrent start, then stop it right away, all it would do would be to poisen the swarm with requests for data thats not needed, and receiving requests for data you wouldn’t be sending.
I will also mention that since you used plural to discribe your downloads, if you run mulitple torrents, your not being a good seeder to any of them. I recommend you run one at a time, or at most two.