In the following image, a nutcracker is floating in space. Attached to points B and C are two thrusters (not shown). Each of them provides an equal force horizontally inwards, shown by the red arrows. As a result, torque will be experienced at the fulcrum (A), and help to crack the nut.
EDIT: it is better to think about this problem without the nut. That way, the nutcracker can "close".
In the next image, another space nutcracker is shown. Here, there is only one thruster (at C). An immovable surface is shown on the left, which is touching B.
The surface provides a reaction force against B, such that the situation is equivalent to the first image.
What I'm having trouble understanding is how this reaction force can provide a torque on the green segment around point A. My confusion arises because in my mind, I have it set that in order for the green segment to experience a torque around A, it must rotate around A. And in order for it to rotate around A, point B must move (in this case, point B must move to the right, while tracing an arc).
But because point B is fixed (the immovable object is always touching B, and the immovable object doesn't move), this can't happen.
I figure it's something to do with how motion is relative (i.e. A moving closer to B is the same as B moving closer to A), but it doesn't sit well with my mechanical intuition of what a torque is.
Can someone help guide me through how to think about this properly?