"As I understand it, when one fills a mold, the chocolate, which is at working temperature, contains predominantly Type V crystals.  As its temperature drops, what keeps the undesirable crystals from forming?"

Nothing.  I bet that was not the answer you were expecting.  Of course, I will explain.  It actually allows me to dig a little deeper into what is tempering.

I’ve mentioned before there is something called degree of temper.  It is a measure of how much your chocolate is tempered.  To review, many think of chocolate temper in a binary sense.  It is either tempered or it isn’t.  Off or on.  0 or 1.  Tempered or bloomed.  And that is a handy model.  But it isn’t the whole truth.  It is more like a number line.  Negative numbers are bloom (crystals other than Type V).  Positive numbers are temper (lots of Type V).  And there is even a zero where there is no crystal structure at all.  Like glass.  It is what we call amorphous.

The higher the number, the greater the temper.  That means that there is more Type V crystal.  If you follow that train of thought that means for some tempers there is something else.  That something else is Types I-IV.   So, see, that is what I mean by ‘nothing’ keeps those ‘undesirables’ from forming.  They do form.  But only in small amounts.  So maybe there is a  better question.

Why don’t those undesirable crystals cause bloom?

Scaffolding.   Seed.  It is the same mechanism that lets seed tempering work.  Namely that if given no direction cocoa butter will randomly form.  If you give it just a little direction, it will stack up appropriately.  So if you have Type V in there, that is the scaffolding that the rest of the cocoa butter will build upon.  In the most technical sense there is some Type IV in there in the tiny spaces that there is not Type V seed.  That ratio of V:IV is degree of temper.  The more IV there is, the softer the temper.  At some point that I don't know, the type IV disrupts the framework of the Type V and you get bloom.

In essence the Type V out competes the undesirables.  With seed the Type V hits the ground running as it were and uses up available cocoa butter to build more Type V.  If everything goes right, it’s is all used up by the time the temperature is low enough for Type IV to start forming.

Since we are talking seed, picture this as a garden.  If you prepare a bed you can either direct seed a plant or transplant a growing seedling.    Experience tells you the seedling will do better.  It has a head start.  The seeds have a tougher go as they have to out compete the weeds.  If you don’t weed then the weeds can win and your seedling dies and your chocolate blooms.  But if you can get your seedlings established and healthy with will use up the nutrients (think free cocoa butter) faster than the weeds and win the race (to mix metaphors).

So your goal is to nuture a good crop of Type V so it can ‘grow’ into healthy tempered chocolate before the Type IV weeds can take over.  You do this by starting with a good amount of seed (seedlings) and keeping the temperature high enough so Type IV can’t form (weeding).

Finally, it is worth noting that sometimes there can be too much of a good thing.  Just like you can have too many seedlings in a given area and you need to thin them for the betterment of the whole area, it is possible to get too much seed in your working chocolate.  If you have been working with your chocolate a long time (it’s all relative) and you notice it thickening up, then that is what you are seeing.  Too much seed.  You need to thin.  And you thin by heating the chocolate up a little bit (just 0.5-1 F for instance).  And just like thinning kills some of the plants you actually want, that heat will ‘kill’ some your Type V.  The result will be that the chocolate will thin back down and you can keep working with it.  You are not destroying all the Type V.  You are just selectively thinning.

There you go.  Tempering as gardening.  I hope that gives you a bit more of a peek behind the curtain.