MACROMEDIA FLASH MX 2004-ACTIONSCRIPT LANGUAGE Reference page 88

The following example converts -1 to a 32-bit integer and shifts it 1 bit to the right:
var x:Number = -1 >> 1;
trace(x); // outputs -1
The following example shows the result of the previous example:
var x:Number = -1
This is because -1 decimal equals 11111111111111111111111111111111 binary (thirty-two
1's), shifting right by one bit causes the least significant (bit farthest to the right) to be discarded
and the most significant bit to be filled in with 1. The result is
11111111111111111111111111111111 (thirty-two 1's) binary, which represents the 32-bit
integer -1.
See also
>>= (bitwise right shift and assignment)
88 Chapter 2: ActionScript Language Reference