I think you are taking it all too literally. If this playing up - as I imagine it - was a Bible passage, it would be told as a parable. It's meant to give a provocative image as a way to convey an intuitive feeling. my guess is, the image to be conveyed is a strong, decisive action onto the keyboard. If you turn your hand palmside up, you find it is easier, especially with the fourth finger, to achieve a strong, decisive action.
On another post, someone remarked that every note played should be preceded by a preparatory motion. They probably meant lifting the fingers up. But as you can see turning the hand palmside up, it is much easier to play when the fingers don't lift up, but play up. Now, if you turn the hand palmside down, imagining you are "playing up," your fingers won't roam around so much, will stay focussed, and can achieve a dynamic, strong, decisive action. This is my best guess and if I do say so myself, I think it's a damn good one.
Imagery shouldn't be taken literally in this case, like in all images that a teacher imparts. Instead, the imagery should be mined for inspiration and for furthering perceptive knowledge about the relation between the body and the keyboard.
Walter Ramsey
PS So far original poster has not said what context he heard this in, so I could be totally off the mark. Was it in the context of the sound, or the physical approach, or projection during performance, or what? Very curious to know.