Complex sentence: An enantiomer can be named by the direction in which it rotates the plane of polarized light, but if it rotates the light clockwise as seen by a viewer toward which the light is traveling, that enantiomer is labeled (+).
Split into simpler sentences: An enantiomer can be named by the direction in which it rotates the plane of polarized light. If it rotates the light clockwise (as seen by a viewer towards whom the light is traveling), that enantiomer is labeled (+).