Perhaps you have watched the impeccable movements of George Szell, the nuances of Seiji Ozawa, the romantic drama of Leonard Bernstein, who flails his arms and occasionally leaves the ground, and been at a loss to see how any musician really knows what the conductor wants him to do, and further, how any musician could possibly change orchestras and not be totally perplexed.
Within the multitude of styles, which are probably as various as the conductors themselves, there are certain basic motions in common.
A conductor generally uses his right hand to keep the beat, a downstroke indicating the first beat of a measure, an upstroke marking the last. A piece in 3/4 time is counted by lowering the arm down, to the right, and up.
It is rarely necessary with a professional orchestra to mark every beat, so a conductor does not keep rigidly to those motions. While never losing the beat, he incorporates a wide sweep of natural expression to bring out his interpretation of the piece.
The left hand is especially important for marking shadings of dynamics and entrances of other instruments or voices. The conductor needn’t cue everyone, however. In a piano concerto, for instance, the solo player undoubtedly knows where to come in, so the conductor might concentrate on bringing in the violins at the right moment, or on toning down the cellos.
Small movements tell the orchestra to play softly, and larger, grandiose gestures obviously indicate a crescendo. A conductor uses short, abrupt motions for fast passages; long, sinuous strokes are appropriate for those to be played slowly.
Most conductors use a baton to keep the beat. Others have recently abandoned the baton to free both hands for evocative gestures. Choral conductors frequently use two hands to keep the beat. Techniques may vary, and musicians may rely a good deal on intuition, but the conductor is essential for coordinating the orchestra and for effecting a single interpretation of the piece.
The system is far from new; Sumerian and Egyptian reliefs from about 2800 B.C. show figures with hands raised leading players of harps and flutes.