Harmonic Syncopation

Hi,

Basically... what is harmonic syncopation?

I understand that it has something to do with the position of harmonic changes in each measure, with regard to the strong / weak beats.
Apparently, it is generally avoided when writing in the Classical period style. So, what are the rules?

My understanding of strong beats within a measure would be 1 and 3 (in 4-4), 1 in (3-4) etc.... is that correct?

Thanks

To paraphrase Huron's "Sweet Anticipation":  harmonic changes on weak beats are nearly always followed by strong beats that also change the harmony.  Harmonic syncopation arises from breaking this learned norm.

Rameau called this a no-no, but then seems to have gone to a lot of effort to call it something else when it happened in music he approved of.  "No, no, that's not harmonic syncopation… that would be wrong if it were harmonic syncopation, but that's not what it is… that's … (finds some convoluted way to analyze the passage).  Since the simplest solution is generally the correct one, I'd say, probably harmonic syncopation is fine, even in ye olde style musics.  And was occasionally practiced.  And Rameau just had a bug up his… um… harpsichord.  

And in case we get too hung up on the idea of hard and fast rules, it's also helpful to remember that Rameau's own compositions were derided as too revolutionary in their harmonies at one point in his lifetime, and as too boring and conventional in their use of harmony at another point, also within his lifetime.  And that the great Bach, (happy birthday, sir!) was actually forgotten for about 100 years until Mendelssohn rediscovered him. We only have 2 and a half of his 5 great "Passions," the St. Matthew and the St. John, and a bit of St. Mark, the rest seem to have been lost forever.  Which tells you something about how little regarded they were at the time.  Take all rules with a grain of salt.  They are attempts to quantify our taste.  And tastes change.  

And since most of you on YC are better theorists than I am, feel free to correct any inaccuracies in the above.  (:

Thanks for the reply! 
This seems to be a 'softer rule' than the counterpoint rules, i.e. parallel fifths and octaves.
Looking back at recent works I've written, I can find quite a few places where I've broken the rule and it doesn't sound 'obviously wrong' in all but the worst offending places..