The short is substituted for the ū of ūh when a preposition precedes it and an affix beginning with y with an indicatory k or ṅ follows.,
As समुह्यते, प्रत्युह्यते, समुह्य, अभ्युह्यते, अभ्युह्य ॥ But ऊह्यते without Preposition. Why do we say of ऊह्? Observe समीह्यते ॥ Why do we say 'before य'? Observe समूहितम् ॥ Why do we say having indicatory क् or ङ्? Observe अभ्यूह्यः with यत् ॥ Of course the shortening takes place of ऊ (or अण् vowel, the word अण् being understood here from 7.4.13), and not when it assumes the form ओ, as आ ऊह्यते = ओह्यते, समोह्यते ॥,