Question about refreshing expired cache in background

[Serhii Korobov]

 Hi developers of Varnish

I read information about Varnish 4 release ( https://varnish-cache.org/li
sts/pipermail/varnish-announce/2014-April/000696.html ) and found there a
new feature http://prntscr.com/k8dgfk . I need an information about
refreshing expired cache in background.

As I know from developer guide, fetching new object in background thread is
available only in Grace Mode - http://book.varnish-
software.com/4.0/chapters/Saving_a_Request.html#grace-mode when object's
ttl has expired but grace period is not over yet.

Here are my questions:
1. When and how can I use background cache refreshing?
2. Why Grace Mode and cache invalidation via "ban" command doesn't work
together?

Thanks
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