While this article is probably not for most of you that read on blogs and forums, sometimes people do not know morale. I have been held up for rulings more than once in tournaments on it's use, so lets get this out on the table. Grab your rulebooks, because I have the page numbers so you can check for yourself.
Taking Moral Checks
Tactics wise, it is valuable to be able to know what 25% casualties means. The rule states pg.44,
A unit losing 25% o more of its models during a single phase must pass a morale check at the end of that phase, or else it will fall back.
This means that "any phase" being movement or shooting. Hence a unit that is falling back during their own movement phase and must fall back through dangerous terrain and takes a casualty or two that ='s 25% must take a moral check at the end of the phase.
Morale while Falling Back
Next, under Morale while falling back pg 46.
Troops who are falling back automatically fail all morale checks, except those to regroup.
Regrouping is done during your movement phase right before the unit moves. This is a special type of leadership test, which falling back units cannot attempt if
1. the unit is below half strength (some units like marines ignore this)
2. There are enemies within 6"
3. The unit is not in coherency
Ok, so in the above example of a unit falling back through dangerous terrain, and takes 25% casualties from the terrain. They must make a check at the end of the movement phase. However, they are already broken and hence automatically fail their leadership test, so they must run again.
Now this applies to a unit that is already broken. So lets look at this a little more closely. Some things force you to make an immediate morale check. Tank shocking, several psychic powers etc.
Tank shock pg 68.
If an enemy model other than another vehicle is reached, the unit must take a morale check and will immediately fall back if it fails it.
Fear of Darkness example pg 63 of Blood angels codex
This power is a psychic shooting attack that hits automatically...... That unit must immediately take a moral test with a -2 penalty to their leadership.
Falling Back More Than Once During a Single Phase
With fear of darkness for example. During your shooting phase you hit a unit with "fear of darkness"... say a unit of nob bikers. They fail their moral check and run. Under morale on pg 43 tells us that if you fail a moral check you run immediately.
Now that they are already fallen back, you decide to shoot at them. You kill 2 of 8 which would force a moral check at the end of your shooting phase. Since they are already broken, they will immediately fail their end of phase check, run again another 3d6 since they are nob bikers, and not the normal 2d6. In essence you have made the nob bikers run 3d6 towards their board edge twice during your shooting phase. They are probably off the board. (this is where you must learn to dodge thrown dice by the ork player and endure rule checks in tournaments)
The goes for tank shocking. you can tank shock a unit more than once during the movement phase, hence running more than once. If you are dark eldar, you can also cause casualties during the movement phase with your reaver jetbikes flyby attacks. Luckily for Dark Eldar you can also tank shock now with your raiders with a 5pt upgrade. Hence you can tank shock people right off the board quickly.
If you have broken a unit during your movement phase, you can also make them run again during your shooting phase. There really is no limit. Jump packers, beasts, and biker type units are the most vulnerable to these type of morale breakers as they run farther.