Some questions
1. What is really cached, when "module cache" is enabled? Module pages?
2. Are where any rules, which can allow rewrite real URL of cached page to URL of object in cache?
3. When cache object is outdated, when it will be refreshed - on first request or automatically after expiration time?
4. Can regeneration of cache produce additional queries for page with cached blocks? (I have 380 queries on uncached page, but sometimes, with block-cache enabled, I see 480 on pages)

1. Module output
2. The cache md5's the requested URL and next time that template is used on that URL, the cache is checked and if it exists (and is within the cached time), the cached output will be served
3. See 2.
4. No, but it will not affect blocks. And vice versa - block caching will not affect module output (and queries used here)

Quote:

Hmmm... please, explain this picture for cached module xhld0 at URL http://modules.xoops2.ru/modules/xhld0/index.php