There are three types of mutations, the nasal mutation, the spirant mutation and lenition.
All the Welsh grammars say Welsh has 3 types of mutation - soft, nasal and spirant , and 9 consonants that are mutable.
The English example provided in the Collegiate is the ‘ch’ in ‘choose’ being a combination of the ‘explosive’ ‘t’ and the spirant ‘sh.’
Despite this assertion I do not believe the spirant pronunciation is obvious at all.
When at the beginning of a word it will be a stop, otherwise it will be a spirant .
The exception to this rule is the preposition a from Latin ` ad ’, where the final d became assimilated in sound with the consonant following it, and then the cluster underwent spirant mutation.
But they did not participate in the later spirantizing and deaffrication processes that characterize the consonant systems of the recorded Iranian languages.
The constraint which forces spirantization also predicts the distribution of released and unreleased stops.
The shifting of the voiced spirants and explosives did not extend over all the HG. dialects.
This paper presents the results of a first acoustic phonetic investigation into voiceless spirantization in the variety of Tuscan Italian spoken in Siena.