speedhack does work.

Only flash 8 to flash 9 use the 4byte *8 method, if it uses AS3 it used another method of storing variables. 

As for an explanation 
100 is normally 1100100 but in as2 it is stored as 1100100xxx, where xxx are control bits that mark specific floating point states, like Infinite, NaN,signed?, etc... 
If these 3 bits are all 0 then the result will be the same as if you multiplied by 8 

Of course, it sometimes happens that these extra bits are not 0, in that case people tell you to do *8+specific value, etc... 

(Copied from the Cheat Engine Forums)