Vaisesika Sūtra 5.1.05

अभिघातान्मुसलसंयोगाद्धस्ते कर्म ॥५॥

abhighātān musalasaṁyogād dhaste karma ॥5॥

[अभिघातात् (abhighātāt) - from impact; मुसल-संयोगात् (musala-saṁyogāt) - from conjunction with the pestle; हस्ते (haste) - in the hand; कर्म (karma) - action;]

The impact is due to the movement arising from the conjunction of the hand with the pestle.