ṣ is always the substitute of the s of skambha after the upasarga (Preposition) vi,
Thus विष्कभ्नाति, विष्कम्भिता, विष्कम्भितुम्, विष्कम्भितव्यम् ॥,
Adhyāya 8 · Pāda 3 · Rule 77
ṣ is always the substitute of the s of skambha after the upasarga (Preposition) vi,
Thus विष्कभ्नाति, विष्कम्भिता, विष्कम्भितुम्, विष्कम्भितव्यम् ॥,