So the prof go to DA without discussing it in the context of MO theory. Instead we got a resonance picture of what happens ... this is what he told us:

1) 1,3-butadiene has both an electrophilic and a nucleophilic terminal carbon atom because we can draw resonance structures showing dipolar terminal carbon atoms ... 

2) For ethene we can similarly draw a dipolar resonance structure. 

From there we simply use Coulomb's law - opposite charges attract - to complete the mechanism. 

So, how adequate is this explanation? Is it a) adequate - probably ... if one simply wants to learn it for a test and b) conductive to further understanding of chemistry? I feel rather short-changed here since I'm paying him the big-bucks (tuition is _insane_) and the book goes into far greater detail ...