Finally, I have found the explanation for the “One-way magnetic force” I found http://pengkuanem.blogspot.com/2013/10/one-way-magnetic-force.html. In fact it is the position of the test coil that prevents it to rotate. In Figure 1, I have drawn the test coil in a horizontal magnetic field B. The forces are acted on the coil’s horizontal side and are vertical.
Please read the article at
Not a “one-way” magnetic force http://pengkuanem.blogspot.com/2013/10/not-one-way-magnetic-force.html
or http://www.academia.edu/4838155/Not_a_one-way_magnetic_force