Piano conducting is not a new thing. Here is what it was like "back then," with quotes from the Grove Music Online Dictionary article on conducting.
Conducting from the keyboard "developed over the course of the 17th and 18th centuries, with the growth of orchestra. Mattheson said in 1739: ‘Things always work out better when I both play and sing along than when I merely stand there and beat time. Playing and singing along in this way inspires and enlivens the performers’.
"The responsibilities of the instrumentalist director eventually extended beyond tempo and beat to include other aspects of performance, like dynamics, articulation, accuracy and affect."
"Several factors made it advantageous to direct from the keyboard: the keyboard player was often the composer of the music being performed; he often held an administrative post as Kapellmeister or Director; and he coached the singers and accompanied them when they sang."
"C.P.E. Bach suggests that if the bass part had long notes, the keyboardist might subdivide them to keep the rhythm going for everyone to hear; he also recommends that the keyboard player raise his hands off the keys between notes, both to produce a more forceful sound and so that the rise and fall of his hands would mark the beat."
"Other authors describe keyboard players marking the beat by bowing at the waist, flapping their elbows, stamping their feet, standing up and waving their arms or even shouting aloud."
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