The mysterious potion

One of the mysteries raised in Harry Potter and the Half-Blood Prince revolves around the mysterious phosphorescent potion in the stone basin at the middle of the underground lake. It's unclear how many layers of magic are surrounding the basin and its contents, but here's what we know:

• Dumbledore and Harry are unable to touch it, as if an invisible barrier prevents them.
• Dumbledore says he is unable to magically vanish, part, scoop up, siphon away, transfigure or charm it in any way, but he is able to easily fill a goblet with the potion and drink it.
• Dumbledore's theory is that it will incapacitate him in some way so as to keep him from getting the Horcrux, but not immediately kill him.
• Upon drinking it, it's not fully clear what Dumbledore experiences, but it's obviously not pleasant. He appears to be in pain and also to be talking to someone who Harry can't see, someone who is making him feel guilty about something. He says he wants to die. He actually does pass out.
• Harry brings him round with the Rennervate spell, though he is still weak. The pressure of a dangerous situation at least twice seems to help bring him back to something close to normal.
• He says to Harry, "that potion . . . was no health drink," and insists only Snape can help him.

Then comes the exchange on top of the tower between Dumbledore and Draco Malfoy. We get some more clues from what Dumbledore says. He's figured out that Rosmerta alerted Draco that Dumbledore had left the school.

"That's right," said Malfoy. "But she said you were just going for a drink, you'd be back. . . ."

"Well, I certainly did have a drink . . . and I came back . . . after a fashion," mumbled Dumbledore. "So you decided to spring a trap for me?"

"We decided to put the Dark Mark over the tower and get you to hurry up here, to see who'd been killed," said Malfoy. "And it worked!"

"Well . . . yes and no . . ." said Dumbledore. "

(emphasis mine)

So, Dumbledore came back "after a fashion". And does the yes and no mean that he didn't truly come back in all senses of the word, or that the trap hadn't worked? Either way, we're left with the feeling that there's more going on than we can understand on the face of it.

My first thought was that we should study potions, especially any potions mentioned in the Half-Blood Prince book. Our beloved author has surely left us a clue somewhere. I'm going to keep a notebook and write them all down when I read the book again, maybe I'll even do it for the whole series. This particular potion must be some complicated Dark Magic.

So what did it do? Did it kill him, and Harry managed to somehow bring him temporarily back to life? So that when Severus did the killing curse, he was only slightly hastening something that would already happen anyway? Interesting thought, but it doesn't sound quite right to me.

However, it seems clear that the potion caused some major change to come over Dumbledore. I really wonder what he meant by the remark that he only came back "after a fashion" -- how was he different, what did the potion do to him?

I guess we have less than 200 days to wait for the answer . . . .