I am little bit confused on how to apply istitoolah characteristic on letter of dhad if it has a vowel...

Question
Assalamuaalaykum warahmatullah,
I am little bit confused on how to apply istitoolah characteristic on letter of dhad if it has a vowel. If it only present on dhad sukoon then why it is chategorized as shifat asliyyah which means Shifat that never leave the letter. Please shade some light on this.
Jazaakallah for answering the question. May Allah give you abundant reward.
Answer
Wa alaikum assalaam wa rahmatullahi wa barakaatuh
The intrinsic characteristics of letter never leave the letter but they are usually more apparent when the letter is saakin.  In the case of the of the letter , the mechanical lengthening is there in all states, but one can feel it more profoundly when it is saakinah. 
The definition of is: It is the pushing of the tongue forward after it collides at its articulation point, and this is due to the influence of the compression of sound on it (the tongue) until the  tip of the tongue lightly touches the gum line of the two top front incisors.
Note: The forward pushing is mechanical; the sound should not travel forward with this involuntary movement.  If the sound went forward with the tongue, the sound of  would then end up sounding like a , which is incorrect.  The lengthened sound of  is that of its characteristic of  and should be as long as  in any other letter that has .