Natyashastra
सूचीमाद्यक्रमं कृत्वा ह्यपक्रान्तं च वामकम् । भुजङ्गत्रासितश्चाद्य एवमेव तु वामकः ॥ ६५॥
By performing the initial movement with the pointed finger, the reverse motion becomes the left position. The first movement, which induces a snake-like fear, also becomes the left position.
english translation
sUcImAdyakramaM kRtvA hyapakrAntaM ca vAmakam । bhujaGgatrAsitazcAdya evameva tu vAmakaH ॥ 65॥
hk transliteration by Sanscript