After the verbs tan 'to expand' and others there is optionally the luk elision of sic when the ātmanepada affixes ta and thās of the 2nd Person follow.,
Thus अतत 6.4.37 or अतनिष्ठ 'he expanded,' अतथाः or अतनिष्ठाः 'thou didst expand.' Similarly असात or असनिष्ट 'he gave.' असाथाः or असनिष्टाः 'thou didst give.'
The roots तन् and सन् must both belong to the Atmanepada, and not to Parasmaipada. Hence in the Parasmaipada there is no elision of सिव्. Thus, अतनिष्ट यूयम् 'you did expand.',