What KM says. Good moderations have the 'real' mgr giving a very brief (1 min/2min) summary of how you performed, whether you exceeded expectations, should be promoted, given payrise or whatever. No-one other than your mgr goes through the actual documentation (unless there is a big humdinger of a row)
Once the summary is read out, interested parties (ie other mgrs that know you, and care) will sometimes probe a bit deeper (as in "Actually I think they did a fab job for me on X and I think they should move up a scale" or "No, they're crap. This happened once...").
You will sometimes get big rows when either two people fundamentally disagree, or when a manager rates all of his own people at one or other end of the scale, or when you get a good HR person in the room who calls out everyone's biases. The ones I've seen generally have the people who don't know you sitting out and enjoying the bunfight, and the people who do know you getting all animated.
For the managers themselves it is a truly horrendous process, but IMHO it is much much fairer than just having one person decide your fate. The very fact that they are having to justify their positions stops managers from just holding back the people they don't personally like and promoting their 'favourite' (although usually there is a lot of political argy bargy going on that can muddy this sometimes).
Anyway, Moderations are a v good thing in theory, don't always work as intended in practice but are certainly better than alternatives.