what little I know in my monkey brain, they are to reduce vibrations.
They spin at twice the crank RPM I beleive, and this is what Unorthodox has to say :
"There is an average of 2.7 HP gained from every pound lost off the crank shaft." - Unorthodox Racing
Id say less then that, but its still a good free mod.
I know lots of DSM's do it.
MonOncTSI anything to add ???