The affix yat comes in the sense of 'what pierces it' after a word in the Accusative -2nd case in construction, provided that it is not a bow with which anything is pierced.,
Thus पादौ विध्यन्ति = पद्याः 6.3.53 शर्कराः, 'what pierce the feet' i.e. pebbles. ऊरव्याः कण्टकाः 'thorns' i. e. what pierce the thigh.
Why do we say \provided it is not a bow, अधनुषा\? Observe पादौ विध्यति धनुषा, no affix is added here. This exclusion of धनुष् indicates that the act of piercing must be such as not to be done by a bow &c. Therefore the affix does not apply to cases like these चौरं विध्यति, शत्रुंविध्यति ॥,