You really shouldn't be doing a "power dive". The air rushing over the propeller is going to impart a pinwheel effect, making it spin faster. If you have the power set to a high level, the Kommandogerat in German aircraft will keep the RPMs high too (reducing prop-pitch with high manifold pressure causes over-boosting and quickly damages or destroys the engine). The Kommandogerat is a mechanical system and probably not "smart" enough to keep power and RPMs at a specified setting when in extreme modes of flight (ie, dive).
Reduce power in a dive; problem solved.
For allied planes you must reduce power and prop-pitch in a dive. However, Il-2 doesn't care if prop-pitch is too low with respect to power. In effect, you only have to adjust prop-pitch and leave power alone.
However, it's not really fun to cheat
and it gives one a better impression of how much easier it was to manage German aircraft engines.
I think DT mentioned that at some point this would be fixed, and that "power dives" won't be possible in any aircraft as they are now (don't quote me on that though).