mechanism in bi cyclic compounds
" What is Carbon tri chloride purpose?" is not answered in the link provided but the answer starts with a carbene attack on furan .

Trichloromethane in presence of a base undergoes 1,1-elimination (or an alpha elimination) because both groups are removed from the same carbon.The electron-withdrawing effect of the halogens makes the hydrogen acidic enough to be removed by a strong base such as hydroxide or alkoxide ion.

A halide ion then acts as a leaving group from the conjugate base, producing the
 carbene.1



Carbenes find a pair of electrons  to complete their valence shell of electrons.carbenes attack a lone pair, a C=C double bond (electron-rich or -poor), or even a C–H bond.In its reactions ,


It is an electrophilic reagent and is known to give ring expansion of aromatic rings.
Electrophillic attack takes place on pyrrole where the intermediate is most stable.


This leeds to normal Reimer–Tiemann reaction product pyrrole 2-aldehyde.

Carbene attack of a C=C double bond leads to an ring expansion leading to 3-chloropyridine.



The reaction of bicyclic compounf acts on similar lines of dichloro carbene attack.2



References 

1 ORGANIC CHEMISTRY ,JOSEPH M. HORNBACK ,UNIVERSITY OF DENVER

2 MARCH’S ADVANCED ORGANIC CHEMISTRY REACTIONS, MECHANISMS,AND STRUCTURE ,SIXTH EDITION