For this feature to work, you'll need to have it enabled. File -> Settings -> Advanced -> Automatically search for alternative download locations.

Basically, the same algorithm as "search for alternates" is used. Since DC++ 0.670, autosearching will use the root hash instead of the file name for its search. After the search is sent, any result returned that has exactly the same TTH is added as a source.

DC++ Change Log Related Entries:
0.800: Do not automatch queue for bad/removed sources (emtee)
0.708: Automatch of search results always on
0.685: Search result automatching waits with match until file list is downloaded if auto-matching enabled; Added customizable maximum user count when autosearching
0.670: Autosearch and automatch queue is now done by tth only (!) (thanks, garg). Automatch search done by tth only, should make things slightly faster. Search for alternates automatically uses tth if available (thanks, garg).
0.4033: Removed autosearch string -- it's not used any more; fixed a bug with autosearch repeating the same search needlessly (this should result in fewer autosearches, good for the hubs)
0.4032: Fixed issue with autosearch not getting filelists; fixed an issue with autosearch by tth not being done
0.402: Automatic search matching is now done exclusively by TTH for those items that have a TTH root and by exact filename for those that don't; matching by name removed for queue items with a TTH root (for autosearches); autosearch is done by TTH for queue items that have a root; fixed a rare deadlock when autosearching
0.307: Added option to match autosearch filenames exactly to further avoid the wrong file being downloaded (default on)
0.306: Changed autosearch so that it only searches if less than 5 sources are online -- this should stop galloping filelist downloads as well
0.305: Added option to automatically download filelist and match it on autosearch match -- this will dramatically speed up autosearching, lessening the strain on the hubs as less searches have to be done.
0.304: Fixed a bug in autosearch when using auto search strings (thanks, ilkka sepp�l�)
0.302: Updates to the autosearch feature
0.26: Search for alternates now sets file type as well (and should work correctly with sizes...)
0.24: Doubled interval between autosearches (2 minutes now, and 60 between each research of the same file) -- should ease the bandwidth load on busy hubs
0.23: Automatic search now searches with type if possible (to ease the load on clients)
0.18: More changes to the autosearch -- it should now be more random and therefore yield better results; removed sources are no longed automatically re-added by the autosearch feature
0.177: Made autosearch more effective (much less hub bandwidth usage...)
0.173: Improved the autosearch algorithm a little bit; fixed search for alternates bug