I've always had problems with the encounter that include clerics and undead due to my difficulty explaining how it works - describing what exactly the cleric does and how the undead react to the cleric's action.
Can anyone explain this feature in a simple way and give a small example of how can I describe to the players what’s happening?
My interpretation is a little bit different, and it relates to why Charisma is the key stat for turning/rebuking undead.
If turning/rebuking undead were simply a matter of channelling divine energy, why wouldn't it key off of Wisdom, the main D&D stat for channelling divine magic? Turning/rebuking keys off of Charisma because the most important aspect to turning/rebuking is the character's force of personality. The divine energy and focus are necessary because it requires that divine energy for the character to do something they normally would never be able to do, which is magically compel undead (creatures normally immune to compulsion) to specific actions.
This interpreation has the side effect of making the turn/rebuke undead mechanic more deity-agnostic, so to speak. A good god or a sun god would obviously devote a portion of its power make the ability to turn/destroy undead available to its clerics; just as a necromantic god would grant the ability to rebuke/control undead to their clerics. But what about all those other gods, neutral or otherwise, whose domains have nothing to do with the undead? Why do they specifically give their clerics the ability to turn/rebuke? The answer, in this interpretation, is that they don't. It is divine power, in the general sense, that allows clerics to do this through their sheer force of personality.
I would then describe the effects of turning/rebuking undead the same way I would describe any other magical compulsion effect. The good priestly character doesn't make the undead run away in fear, they command the undead to leave this area at once. And through the combination of the cleric's sheer force of personality and the source of their divine power, they obey.
Note this is by no means RAW (if it were the PHB/SRD would refer to the effect as a compulsion, and it also specifically states that it involves channelling positive/negative energy) but it is the way I have always interpreted it based on the reliance on Charisma.
I don't think there is a rule that describes exactly how this works, so the following is interpretation.
Well, in order to Turn Undead you need to "brandish your holy symbol". The way I've always explained it when it came up is that Undeath itself is an abberation: it's magic reanimating that which should be dead. There's no natural life there, just an evil mockery of life.
By raising your holy symbol and invoking the power of your Deity, you're attacking the magic that keeps the Undead animated with positive energy. That effect terrifies the undead, and they flee as fast as they can. If you disrupt it enough, you can outright undo it and destroy the undead instead.
Some gods find undeath more abhorrant than others. Pelor for example has this line in Complete Divine (p 117):
More so than the adherents of any other faith, the followers of Pelor find themselves striving against the Undead.
That's probably the reason why Pelor has the Sun domain, whose power lets his followers destroy Undead outright, as well as the Radiant Servant of Pelor prestige class, which lets them use that power multiple times a day.
Other gods don't mind undead so much, and may see them as useful tools or even a good idea. Those ones tend to be the evil ones that can rebuke and command undead instead. Then you have the ones that don't care, and they simply let their Clerics decide how to best handle the undead (to be turned/destroyed, or rebuked/controlled).
Given all that, when I was playing a RSoP Cleric and turning undead, I described my turn as something like this:
Stepping to the fore, I grasp my symbol of Pelor and hold it before me. Then I pray "Light of the Shining One, come forth and purge this evil from the land!"
And then the GM might tell me how the divine light of Pelor shines from me and destroys the undead... or he might just tell me to make the attack roll. ;)
Like Tridus, I cannot give a official explanation on how it works because there is none detailed enough ( I am not talking about the mechanics, but the theoric base of how it works). I can, however, give the one I use when asked:
A cleric using its turn undead ability, depending on his patron deity and/or alignment, gains the ability to channel either positive energy or negative energy.
The positive energy is linked to life, and is the force that powers up healing magic. Negative energy is linked to death, and powers up most magicks tied to necromancy.
Undead beings are closely tied to negative energy also. Incorporeal undead are souls that refuse going to their final rest, and corporeal undead are corpses reanimated to a mockery of life, some of them devoid of sentience save the most basic instincs, while others still retain its original, albeit greatly corrupted, soul. In all cases, negative energy is the power that allows them to continue their antinatural existence, using it to forcefully isolate themselves from the cycle of life and death.
When a cleric channel negative energy into a undead creature, it is overcharged with negative energy, producing in it the equivalent of what a intoxicated state would cause on a living being. A ligth intoxication merely confuses and disorient the creature, making it stop and cower in awe. A heavy dose of negative energy completely overloads its system, making it controllable. Unintelligent undead are simple hijacked, their basic instincs overwritten with the orders of the cleric. Intelligent undead become instead vulnerable to suggestion, as the clerics commands are implanted in their psyche through the link created by the energy transference.
Channeled positive energy, on the other side, has dramatically different effects. Minor or medium doses of life-giving energy weakens the barrier that isolates the undead from the natural order. This makes the undead become alive for a brief moment. The shock caused on a soul that return to a cuasi-living state (non-sentient undead gain some sort of animalistic sentience), when confronted with the reality of its existence as a undead being, overwhelms it with a mix of fear, sorrow and despair, making it flee the source of its pain: the cleric. A really big dose of positive energy completely breaks the barrier, returning the affected undead to the natural order, and thus destroying it.
The description here seems to sum it up nicely.
http://www.dandwiki.com/wiki/SRD:Turn_or_Rebuke_Undead
Good clerics and paladins and some neutral clerics can channel positive energy, which can halt, drive off (rout), or destroy undead.
Evil clerics and some neutral clerics can channel negative energy, which can halt, awe (rebuke), control (command), or bolster undead.
Regardless of the effect, the general term for the activity is “turning.” When attempting to exercise their divine control over these creatures, characters make turning checks.
emphasis added by me.
