How to set CPU frequencies lower than 400 MHz
Unless CPU tuner operates in poweruser mode it hides frequencies below 400MHz from the user. To activate poweruser mode, in the Settings tab set experience level to power user.
Or set Min displayed frequency to the desired value. Be aware that frequencies below 400MHz might deadlock your phone in sleep state. You then can't switch the phone back on again and need to unplug the batter..!