for the AX-12a servos the part that lets you feel haptic feedback is
x=actuator._get_current_load() in python where actuator is servo where you are getting the torque from and actuator._set_torque_limit(x) where this actuator is the new one that you are trying to feel the feedback on if you look at the data sheet here it shows you a list for the servos
Area Address(Hexadecimal) Name Description Access Initial Value(Hexadecimal) the chart with the titles above is the one I am taking about.
the _set_torque_limit is the 34 torque limit you are setting the torque limit low because that is the minimum torque you can feel on the servo.
the _get_current_load goes with the 40 which is current load this tells you what torque the motor is feeling.
this is also very easy to do with C or C++ there is a preset up just like there is with python