Hi EtudeInGold
I know this piece very well. The particular passage you describe is an absolute beast,Â

 unless you have huge hands (and I do not). The technique is hard to describe, but I will try.
The secret to playing the lh is not to try to hold your wrist still - this just leads to tension and uneven playing. Take the first group of semis B F# A B. Your wrist needs to start quite low, then lift slightly and move towards the right as you play the F# and B. If you get this right then it will be slightly leading your fingers in the direction the notes are travelling.
Come the 4th semi, the B, your wrist should still be leading your hand but needs to fall in preparation to going back to the starting position for the first note in each group.
By the time it finishes each group of semis, your left wrist will have followed an eliptical circle, rather like a planet with a very long orbit round the sun. This is horribly complicated to describe, so I hope it makes sense. Do say if it does not and I will have another go.
The rh here is just a matter of careful fingering and slow practise, although playing around with the rhythm also helps a lot once you know the notes - look further down Viva Piano to one of my replies to Fred ('Speed vs Accuracy') to find the location of a description I posted recently on how to use them.
Hope all this helps
SteveÂ

that helps hugely! (yes, i only recently passed my Grade 5, studying 6/7, but i need SOMETHING to keep me entertained and this movement i LOVE)