harry potter six- Sectumsempra?

There isn't an actual name for the counter-spell used to stop Sectumsempra. In the movie, it sounds like he's saying something, but until the movie is on DVD, or you find a playable version with subtitles, we must assume that what he says is gibberish.

In the book, it describes what Snape says as "an incantation that sounded almost like song", but gives no more detail than that. You need to use it three times in succession, until the wounds practically vanish. Then you need Dittany to prevent scarring.

But to answer simply- there is no name for the spell used to reverse the effects of Sectumsempra.

Sectumsempra is a Dark curse invented by Severus Snape (under his alias of "The Half-Blood Prince") for use against his enemies, and it soon became one of his specialties.

The curse also has a counter-curse, most likely also invented by Snape, but all that is known is that it is "an incantation that sounded almost like song". The first time it is used, the flow of blood eases up; the second time causes the wounds to knit; the third time removes the worst effects of the curse near-instantly. The victim would still require medical treatment, and if Dittany is applied immediately, scarring can be avoided.

he didnt say, he said it in his head

OMG,Nerds.

i think he mumbled stuff...i couldnt quite hear it in the movie! but in the book he just mumbled somthing too?2 idk!?