lightens the load on the crankshaft so it can rev it more easily and quicker. in theory and when it is balanched should make it run abit smoother also in theory
The way my dad did it on all my pit bikes was to undo the nut right till the end of the thread so it is on a bit. Get 2 tyre levers/ flat head screw drivers (big ones) get some one to put pressure as if you was to pry it off and hit the nut on the end really hard and fast with a rubber mallet and it should just come off. He could do it in like 30 seconds...
Well not really.... Did it loadsss of times. My dad has been stripping engines since he was like 15 and is now like 52 so I think he knows what he is doing....