QUOTE(frmain @ Jan 1 2008, 01:58 AM)

They do the thing with one hand underneath and the other hand going on top back and forth. How would I be able to tell how to do that from the sheet music, and is it necessary to use that exact hand movement?
Generally speaking, Anything Debussy writes in the treble clef is for the right hand, and anything in the bass is for the left. If both hands are written on the same stave they are usually written as clearly separate lines. Debussy uses both these methods in "The snow is dancing".
Occasionally a figure to be played by a single hand is split between the two staves. In those cases the notes in the group are usually scored as a connected group, and there is usually no choice about which of the hands takes the group, as the other will be busy elsewhere.
Where there is variation from this basic scheme Debussy usually marks it explicitly by marking the relevant notes or passages as mg (main gauche) for the left hand and md (main droite) for the right. When he doesn't do this, he has other tricks to indicate crossing hands.
Sometimes he will score a note in the bass clef for the left hand (and similarly in the treble for the right) where it would seem easier to take it as the continuation of a run with the other hand. e.g. he does this on the first page of Dr Gradus.
Another technique he uses is to score a note that is part of a group on the same stave (treble or bass) with the tail going in the opposite direction to the rest of the group. This is an indication that you should consider taking the note with the other hand, though it may also indicate simply a note of a different time-value to the rest of the group, to be played with the same hand, but held. There may also be some special mark above an affected note, staccato, accent, or tenuto.
These latter two techniques are usually unambiguous - the hand that does the crossing was doing nothing anyway - or the whole section is washed in pedal, so there is no need to hold notes for their full length with your fingers. But now and again they raise another problem. Often, to be able to cross hands, you have to release a note before it has had its full value.
Sometimes, because of the spread of notes there is no choice, but there are also cases where you could play all the notes, their full values, without crossing hands, but crossing hands helps the flow and accentuation in other ways, and is desirable for artistic/interpretative reasons. In these cases you have to make a musical judgement. Will crossing hands enhance the musical effect (bearing in mind that you will have much better control of a single high note taken with the LH) sufficiently to compensate for any compromises that it requires in other areas - or is it just a conventional flamboyant gesture that you don't really need?
Whether you are forced to cross hands, or choose to do it where it is not absolutely necessary, it can mean that the exact realization of the sound (note lengths) indicated by the score is not possible. You have two choices. You can release the impossible-to-hold-for-their-full-length note(s) early and accept their absence from the sound, or hold with the pedal, and accept the sound of other notes that get held as well, along with all the extra harmonics that are created. If you have a 3-pedal piano you have a third option. Catch the note(s) that would otherwise be released early (usually in the bass) with the third pedal, and play the rest as dry as you like.
Regarding the hand movements. In any kind of big leap, including crossing hands, it is safest to break the movement into two components:
1. Getting the hand into position
2. Taking the notes
When you get up to speed it looks like one continuous swoop - but if you start practicing it slowly as one swoop, taking the notes at the end of a graceful arc, then you are likely to hit a lot of wrong notes when you step up up to performance speed.
In extended sections where the two hands are playing in the same register, as in "The snow is dancing" it is often difficult to decide which hand should be on top. Sometimes the right way round is forced on you by where the hands have to move to when they separate. Sometimes it is forced on you as the only way to take the notes without the hands getting in each other's way. Often there is more than one way to do it, so if whatever comes naturally to you creates the right musical effect and is secure then it is probably OK. If you are stuck then watching how a good pianist does it and copying is sensible