It looked different? Maybe because they're higher-level demons? But I thought the point he tried to make when Dean caught him at it, early on in S4, was that he didn't kill the host, but he did kill the demons. I could be wrong, though.

I'd have to look at early S4 to figure which ep features the sinking black cloud/ash, etc.

I thought there were three scenarios: cloud in vent means the demon is under its own steam and will possess again; cloud to floor means demon to hell and it will be a bitch for it to come back (Sam and exorcisms); internal light show end of the demon road (kill shot from Colt, knife, or hopped up Sam).

Approximately.

Now that you put it that way, you may be right.

Mommmmmmmm! ita's making me think!

I know the Great Pumpkin episode featured Sam HaneSamhain (aghhh, it burns!) billowing out of his host and burning to ash around the body.

ita, I think the first two are the same, actually. An exorcised demon cloud looks the same, and often goes up, as a voluntarily exiting one. YED voluntarily left John's body and seeped through the floor of the cabin.

Sam pulling demons left the hosts alive, and left the demon a smoking puddle of black goo, with flickering embers here and there, on the floor. No return to hell to rise and possess again.

Then Alastair's challenge to Sam to send him back to hell and Sam's assertion that he can kill now doesn't make sense.

Maybe Alastair just mistakenly thought he was too strong to be destroyed that way, as with his immunity to Ruby's knife?

Sam's reply was in line with that assumption, though, and what he did to Alastair was different from what he'd been doing up until then.

Now that I'm home again, I went to the Supernatural wiki, and their assumptions line up with mine WRT Sam:

Sam can force a demon to leave its host through sheer force of will. Initially, using this ability caused Sam a great deal of stress and would often result in severe headaches and nosebleeds, but with time and practice he was able to exorcise a demon without these side-effects. Once removed, the demon "smoke" falls to the ground and burns a black circle as it descends back to Hell.

...

When fueled by a certain amount of demon blood, Sam powers are strong enough to allow him to not only exorcise demons, but kill them too. Killing a demon creates the same light inside their body as when they're killed by the Colt or Ruby's knife.

It makes sense laid out like that. Otherwise what he said to Alistair would have made none.

And tonight we can be broken by My Bloody Valentine again! Whoo.