involves slightly porting and lining up of the plenum to lower inlet and lower inlet to the block, resulting in better air/fuel flow into the block, giving slightly better mid range torque, normally also involves fitting a ptfe gasket to reducing heat transfer from the block to inlet/plenum. might also be better to be used in conjuction with a ported throttle body.
i got mine done at jamsport in nottingham when it was first being done on the 197, i beleive more places may do them now, and maybe on an exchange basis.