the king shouted and sauron lifted his arms up in suprise...and accidentally killed isildurs dad...then sauron was like upset so he walked over to see if he could help....but being all metal isildur got there first
sauron said sorry and put his hand out to lift isildur up because he was getting muddy...and bloody....and muddy and bloody rhyme so sauron was scared all the watching elves would make a poem or something....bur isildur was so into elronds singing voice, and wanted to marry elrond......he couldnt afford an engagment gift though....and he saw sauron...and cut off his ring.....and then elrond and isildur went into the 'cracks of doom' (less said the bnetter) but then made up an excuse as to the murder of me...and the 'incident' in the mountain
and that is the story we have come to learn......
while were on the subject of 'why' questions...why didnt all the uruks just fire arrows at boromir....MUCH lower death toll.....and 'why' in the film was there a door into orodruin....if thats the only way in..to the only place your one means of survival can be destroyed you wouldnt have a big door there
.............
That was an optical confusion for the audience, cut cunningly by PJ. The shot is from an earlier thing when Sauron crushes Gilgalad, and in fact Isildur never cut Sauron's ring but PJ wanted us to believe so so he cut together other scenes, like this one or the one of his helmet falling (lucky camera man or reporter actually filmed that some ages earlier... ehem... after a battle... sauron was returning home and wanted to have a shower so he put off his helmet. The scene with the ring itself is totally computeranimated and an invention of PJ to make the whole thing more conviencing.