All of the supported BitTorrent applications have a web interface that allow other applications to communicate with it using an API. More specifically, it supports the JSON-RPC or XML-RPC protocol for easy communication over the internet. With the web interface you can view the status, pause, resume, remove or add torrents. Transdroid uses the exposed Transmission API, that of µTorrent and that of Deluge as well. Via asynchronous HTTP web requests and responses, you can fully control the daemons.
The isoHunt-powered search is build using the same approach. isoHunt exposes a JSON-RPC API as well.