Hand & forearm clusters

[pianovirus]

Please share any kind of hints or experience with this technique.

For example, I'm unsure about the orientation of the forearm during (arpeggiated) clusters on both black and white keys. Should the wrist and hand appear vertical ("thumb upwards") or rather lie flat (thumb horizontal) on the keys?

Some other things I'm finding not so easy as a cluster newbie:

• getting the boundary right (i.e. cluster range not too short or too long)
• really hitting all (both black and white) keys in the range. This is crucial if some (again both black & white) keys must be silently depressed with the fingers while the cluster sound is held with the pedal so that after release of the pedal only the sound of the few notes held with the fingers remains. If one black or white key is missing it will be painfully obvious in the "thinned" sound.
• getting smooth (upwards or downwards) arpeggios (again over both black and white keys with a single forearm)

[pies]

a

[pianovirus]

Thanks, this is really good stuff.
(any other perspective on my questions would still be interesting of course)