Well, according to the wikipedia article, it goes to the extent that even if you've got evidence through, say, coercion, while that evidence would be invalid, you could use that information so obtained to collect further valid evidence, or even just get the suspect to repeat the previous statements (not under coercion) then resubmit it.

But I've mostly just got this from my wife complaining about it. There's very little structurally in place to prevent the police from abusing their power in germany, and when it comes to evidence collection, they are definitely incentivized to do so.