And it's grayed out (not user-configurable), at least here with the DZ68BC. The documentation makes it seem like enabling it would enable a deeper S4/S5 (that doesn't quite make sense to me, since S4 and S5 shouldn't have fluid meanings) but at the cost of being forced to use the power button to wake it.
Deeper sleep reduces the standby current drawn. When enabled the USB ports are powered off and unresponsive, this saves some power however it means you can't bring you computer out of sleep by hitting a key combination on the keyboard, as the keyboard is off and unpowered.
Perhaps it is disabled because you have set the option to wake the computer using the keyboard?