Hakushu (拍手) refers to the ceremonial clapping practice in Shinto, also known as "Kashiwade." This form of clapping is quiet and gentle, and unlike western applause.
Its origin is unknown. Some people think it holds a Buddhist meaning where the right hand symbolizes Buddha, and the left hand represents all beings, thus signifying the unity of Buddha and all beings.
Due to Shinbutsu-shūgō the origins of this practice are unclear. Shinto and Buddhism have been together so long that many things could come from either. The origin must be ancient though as it is found in the Wajinden describing the Yayoi rituals.
This tradition can be observed in martial arts like Aikido and Bujinkan today.