It works by creating a redirect with its own breadcrumb trail, different from that of the target article:
#REDIRECT [[Russia]] {{isPartOf|Asia}}
If an article indicates that Siberia {{isPartOf|Russia (Asia)}} the breadcrumb trail becomes " Asia > Russia (Asia) > Siberia " instead of " Europe > Russia > Siberia "
This appears not to be in use as of mid-2013 (see discussion at Talk:Asia#Proposed Eurasia breadcrumb), but should be either kept intact for documentation purposes or deleted outright. Keeping the redirect minus the breadcrumb entirely defeats the purpose for which this was proposed and attempted. K7L (talk) 11:14, 10 August 2013 (UTC)