Of ec vowels, ik is the substitute, when short is to be substituted.,
This sutra points out the हृस्व substitutes of एच्. We know there are short and long vowels in Sanskrit, but properly speaking the एच् vowels have no corresponding short vowels. Therefore, when, in any rule, एच् vowels are told to be shortened, this sutra declares that the short vowels of ए and ऐ, ओ and औ, for the purposes of the rule, will be इ and उ respectively. Thus, 1.2.47 declares:-- \the short vowel is the substitute in the neuter of a crude form provided it end in a vowel.\ Therefore in compounding अति + रै, the ऐ must be shortened. Properly speaking ऐ has no short vowel corresponding to it, but by virtue of this sutra, इ supplies the place of such a short vowel, and we have अतिरि 'extravagant' so नौ, अतिनु 'disembarked or landed,' गो, उपगु 'near a cow.' All avyayibhava compounds are neuter 2.4.18.
Why do we say 'of एच्'? Because the short of other vowels will not be इक्. Thus the short of आ is अ. As, अति = खट्वा = अतिखट्वः, अति + माला = अतिमालः ।
Why do we say 'when short is to be substituted?' Because when प्लुत or prolated vowels are to be substituted for एच्, the इक् will not be the substitute. As देवदत्त O Devadatta ! देवद३त्त !,