Reverse controls which group of atoms will move (on one side of the rotatable bond or the other) during a rotation. When reverse is invoked, the portion of the molecule which rotated previously (as delineated in the rotation command description) remains fixed and vice versa in subsequent rotations. Also, the rotation angle is reset to zero degrees.
Warning: In assigning multiple bond rotations to a model, two rotations cannot affect a common set of atoms unless one affected set is a complete subset of the other. That is, rotations must be properly nested. In executing reverse, reversing the bond rotation may cause such a conflict and be disallowed. Reversing bond rotations must be done in an appropriate order such that all intermediate combinations are legal (see rotation).
See also: rotation, the Adjust Torsions panel