I witnessed a few of the Elvo-RC-therapy-sessions, so I will have a go:
Imagine 1 hole in the piston with 2 90 degree edges. When the piston moves veeeery slow through the holes, the oil coming from the sides needs to go around the 90 degree corner. Because it moves slowly, it can do this without leaving the piston's surface.
When the piston moves faster, the oil moves faster, and the oil from the sides has more difficulty taking the corner: it "rounds" the corner and 2 vacuum pockets arise. As a consequence of this rounding, the hole size that can be effectivelly used narrows, and is at a minimum half way the hole.
When you chamfer the 90 degree corner into 2 45 degree corners, 4 smaller vacuum pockets arise instead of 2 . The narrowing effect of the hole is less now.
Till here, pure pack, I think it's more inertia of the oil, and not turbulence, as Elvo mentioned.
So in theory, chamfering only the top or only the bottom results in the same pack, because the effective hole size is the same, only the minimum diameter is positioned a little higher (only bottom chamfered) or lower (only top chamfered) instead of half way the hole (none or both chamfered).
But as Flipside mentioned, it clearly does not feel the same both ways when you chamfer 1 side of the holes! The damper feels softer when the oil enters the chamfered side.
The cone-shape of oil that is created at the chamfered side will probably create a more efficient flow through the narrowest point, because the pressure can build up nicely in the cone. When the oil needs to come from the unchamfered side, most of the oil that was in the helping cone before, is pushing against the piston now, so resisting instead of helping. But more important, the "guiding" cone is non-existing, so the flow at the narrowest point will be less effective.
So when explaining chamfering, then I think you could call it a sort of laminar/turbulence at play, like Reflexuk did.
Why a Ghea works the same with chamfered piston side on top or bottom, I really don't know. Like Flipside, I find that hard to believe to be honest. Maybe not noticable (I dont know how much oil passes via the sides compared to through the holes...), but there must be a difference.
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