Like they said, Miyamoto wanted Link to raise his sword high while riding on horseback in TP. They did that, but it didn't quite have the effect they were looking for.
We already know that raising the sword to the sky charges it, and we know that there is a connection between the heavens and the sword this time around. The title "Skyward Sword" isn't just referring to the correlation between the two, though. The image of holding up a sword to the sky is a very powerful and symbolic image. It's empowering. It gives hope and shows triumph. Doesn't anybody remember Miyamoto holding up the Master Sword some years ago, hyping up all the fans? Military and geurilla leaders did/do it to pump up their soldiers. Even today, people raise their guns to the sky. The hero holding his weapon to the sky.... very moving.
As for how exactly the sword gains power... dunno. I want to think goddesses, but this Mysterious Blue Figure looks more like a fairy... still, she doesn't look human, so she could be a goddess or lesser spirit of sorts. Maybe the "charge" is a blessing. I don't know. It does appear to be the Master Sword, though, sooo... I really can't think of what this charged power might be other than a blessing of sorts. Or.. perhaps it draws power from the heavens? Like, actually uses the power of higher beings?