[John Pancoast]

Stewy1, another minor tip involving snapshots. If you've had to go defensive against another sub, firing off a snapshot before doing so is generally a good idea. Also, always enable a snapshot right away; don't wait for it to reach it's rte distance. Always have a torpedo set up for a snapshot beforehand. You don't want to waste evasion time while defensive setting it up.
It gives the other guy something to deal with while you're trying to defeat his torpedo.
Otherwise, he can just continue to drive in for the kill, unconcerned. You're not necessarily trying for a kill, you just want to keep the other guy busy too.
It's an old flight sim (maybe real life too, don't know) tactic in a bvr fight against a better combat jet and it can work with subs too, with the added benefit that acoustic torpedoes tend to be more dangerous/harder to shake than any jet missle for various reasons.
Also, Kapitan has some great advice. To add, when defensive you're also trying to have the torpedo use up it's energy. You have an unlimited supply of it, the torpedo doesn't.
That is another (though minor) reason why you do the maneuvers he listed.