t is substituted for the final s of a root before an ārddhadhātuka affix beginning with a s,
As वत्स्यति, अवत्स्यत, विवत्सति, जिघत्सति ॥ Why do we say 'when ending in स'? Observe वक्ष्यति ॥ Why do we say 'beginning with स'? Observe घासः, वासः ॥ Why an ardhadhatuka? Observe आस्से, वस्से, from आस् 'to sit' and वस् 'to cover'; both Adadi roots.,