Understanding ... RewriteCond foo#%{ENV:bar} ^([^#]+)#\1$

Understanding ... RewriteCond foo#%{ENV:bar} ^([^#]+)#\1$

I am trying to understand what this rewrite condition exactly does:
RewriteCond foo#%{ENV:bar} ^([^#]+)#\1$
As stated in "Using custom environment variables in .htaccess" this could
be used as a workaround for testing environment variables in a rewrite
condition - e.g. that "foo" equals the value of bar.