Like most things with the algorithm I can only relate indications. Over the recent updates forums have been stuffed with people screaming they just lost their positioning colossally, on investigation this is because they have a ton of spun article spam out there or similar which Google is now definitely against, so their position has plummeted.
Of course, this could be because they're getting "punished" or it could be because the "bonus points" they were getting for those spam links are no longer earning bonus points.
There's not quite as much forum traffic or blogs around about it but I've seen a few stating examples of how link disavow has helped dig many of these spam linked sites back out of the basement; they disavowed the bad links and their search position went back up.
Google aren't going to give you bonus points just because you disavowed a bad link, so it's logical that disvowing a bad link removes some negative "punishment", allowing these sites to bounce back up to a healthy position.
Perhaps it does seem strange people aren't constantly crashing each other's sites, but then it's a bit like nuclear deterrent isn't it, if someone did it to you you'd probably do it back because it is easy to do.
I'm not saying this is a good thing, it just seems the only thing that makes sense. I know some updates are purely "remove-the-bonus-points" based ie EMD, but I can't see any other explanation for the disavow bounce effect.