Tobar said:

I had always thought it was his hand. But according to the script:

There is an ominous cracking sound from the base of the weather vane and a piece breaks off, falling into the clouds far below.

I always thought it was, if not his hand, his lighsaber (the thing falls fast).

Though it can be his hand still holding his lightsaber... But, in the last case the lightsaber shouldn't be switched off...

The only thing that bothers me anyway in all this is the shout of luke when he has his hand cut. I'm sure any one of us would have no time to feel or realise anything if a lightsaber stroke cutted our own hand...

... Hey Ripplin ! How do you say:"J' en mettrais ma main à couper" in English ?... :)