Do you want clients to see the change of the host and/or URI in the browser address bar? Or do you want to rewrite it transparently? Redirects will show the client the change. Rewrites will hide them. The ProxyPass rule uses rewrites.
The rewrite condition is checking of the referer starts with https://host1.domain.de/. The request rewriting is replacing /test/ with http://host2.domain.de/some/path/. The response rule is doing rewriting /test/ to http://host2.domain.de/some/path/.
Also, do you know if there is actually response headers (mainly redirects) and/or payload content that you want to rewrite from /test/ to http://host2.domain.de/some/path/? If so, you can use an iRule like this to rewrite the redirects:
http://devcentral.f5.com/wiki/default.aspx/iRules/RewriteHTTPRedirectHostname.html
And you can use a blank stream profile and STREAM::expression iRule to rewrite the response content:
http://devcentral.f5.com/wiki/default.aspx/iRules/stream__expression
Aaron