In the projections of Newman, we are looking for the C2-C3 bond. The C2 bond is in front and it has two attached methyl groups; C3 is behind C2 and it has only one attached methyl group. The angle of internal rotation also called torsion angle or dihedral angle indicates the rotation of degree about the two bonds (C2-C3) rather than the rotation of the entire molecules. Because the x-axis indicates the angle of internal rotation, looking sequentially from conformation to conformation (i.e., A to B to C, etc.) should convey sequential "snapshots" of one full rotation - in 60° increments - about the C2-C3 axis.
