Tautomerization:  a change in position of a hydrogen atom or a proton, paired with a shift in the position of a pi bond. 

In this mechanism the Imine has a hydrogen at the carbon alpha to the CN bond.  This proton is lost to create the enamine.

Here's the mechanism of imine-enamine tautomerization:

-This reaction is catalyzed by either an acid or a base. 

-The intermediate shown here is called an iminium ion. 

-The imine tautomer is favored over the enamine tautomer except when there are very large alkyl groups on the nitrogen.

Enamines are important intermediates when ketones are reaced with ammonia and primary and secondary amines.  They serve as good carbon nucleophiles:

                                                                          Enamine

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