How to hold an object with one hand and turn the top with the other hand

Hi, I have a motor connected to a servo arm using a few screws. There is a servo horn in between the servo and the servo arm. In the model with only the servo and the servo horn, I can turn the horn. However, after I inserted the motor+horn into the assembly and then connected the component to a servo arm, I can't seem to be able to turn the servo arm. To test, is there a way to temporarily locked the location of the motor and use the mouse to drag the servo arm to see if it turns? This is similar to holding an object with one hand while turning the top of it with the other hand. Thanks.

SolidworksGeneral