When I turn off a servo how much do torque I need to rotate it?
Servos such as you link to are absolutely NOT suitable for throttle activation on a vehicle (if that is your application). For a start they are highly geared down, so turning the motor by putting force on the output shaft can easily damage the gear train and lock it up.  

There are commonly three ways to do throttle activation (and I've designed Diesel alternator throttle controls in a past life).


Vacuum activation with a PWM vacuum modulator solenoid
PWM solenoid (this is what I used for constant rpm applications)
Motor/clutch assembly (vast majority of today's vehicle cruise controls)


If you want to attempt your own throttle control system you can buy vehicle rated throttle servos (for cruise control) very cheaply (~$50-150 new) which starts right about the same price as the Dynamixel servo alone, but already has the cable pulls attached.
You can get used cruise control servos for &lt; $20 on Ebay, so why not use the real thing.