The affixes ktavā and ṇamul come after the root kṛ 'to make' when the word triyak is in composition in the sense of 'carrying to the end'.,
The word अपवर्ग means 'completing or finishing'. Thus तिर्यक् कृत्य, तिर्यक् कृत्वा or तिर्यक्कारं गतः - समाप्य गतः 'having completed, he went away'.
Why do we say 'when meaning completion'? Observe तिर्यक् कृत्वा काष्ठं गतः 'having placed the wood obliquely, he went away'. Here णमुल् is not employed. See 2.2.22 for compounding. The word तिर्यचि is the locative of the word-form तिर्यच्, and not of the word तिर्यच् । Other examples are: एतदोश् 5.3.5; अदसो मात् (I. I. 12).,