Example A 

This  Is a serpentine style core like CoPar uses. The Fins go into a Zigzag fashion. The fins are the strips of copper between the tubes. In this picture you can see the fin and tubes clearly. The serpentine style fin has louvers in them, to disrupt air flow to cool better. The number one problem with the louvers is they do clog up with debris and dust. When the fins and louvers gets full of debris and dust, this causes the Hi-Lo to get hot.  While the temperature rises, it builds presser which causes a failure in either the tube or the tank.

This shows the louvers.....the ones that get clogged....

This is a clogged radiator 

Example B

This example is the Flat-Fin style core. The fins are dimpled not louvered, this allows the air to be disrupted but since there is an indent and not an actual hole they can not clog or fill with debris. The construction of this type of core is that the tube goes threw the fin which increases strength. 

In this picture you can see the dimples in the fin.. If you choose to use shop air to clean the core by blowing it out there is nothing to restrict air flow allowing you to clean this unit of debris...