The HSE is two wheel drive hydro, the HSD is 4WD hydro and the HSDB is 4WD with bi-speed option. The bi-speed turning feature spins the front wheels faster to allow easier turning while the FEL is loaded. It is nice but not really that big a deal. Like I said, it does reduce steering effort which is nearly effortless anyway and slightly decreases turning radius when the FEL is heavily loaded. Good luck. J