"The torpedo could be programmed to run in a straight line for a certain distance, then if a hit did not occur, the torpedo would reverse course turning right or left (as programmed) and ran for a further 800 or 1,600 meters before reversing course again."
Source:
http://www.uboataces.com/torpedo-tiiia.shtml
BTW if you open your torpedo attack map (not the navigation map but the other where you have all the torpedo settings available to you) you can see the torpedoes expected track so you can understand much better what each setting does.
It's an excellent weapon for later years when attacking an escorted convoy from around around 2000 m is suicidal. If your solution is at least within the ballpark you're bound to score a hit.