They way the arm is structured make increasingly difficult to turn the arm out of position, I mean, the force needed increases as you try to overcome the motor force because, by design, (intended or not) the distance of the effective lever increases as you try to close the arm.
Besides that, even when the force may not be the best attainable, It may still be more than enough.
No, I'm wrong.
The symmetry of the design make it work ok, as the length of the string is constant. If the outer bolts are placed in another position as drawn, they will separate more quickly than the inner bolts can get closer, inducing slack to the string.
The issue here is that, by the "disposable knife design" the inner bolts cannot get closer, due to the extended mounting points, while inthe other hand, the outer bolts can get very close as there are no extensions on their side.
So, if anyone would like to increase the torque, the mounting point of both bolts shoul be moved symmetrically, to maintain a constant length of string.
Please see this diagram:
L1 and L2 have to be equal so AC and BD add to a constant number.