Please explain why the letter 'dhaad' is characterized as a letter of rakhaawah.

salamu alaikum,
Please explain why the letter  'dhaad' is characterized as a letter of rakhaawah.  To me, it seems that there is a mild running of the sound, which ends shortly thereafter, similar to the 'ain.  It is very hard for me to keep the dhaad at its makhraj and let the sound run for long.  After a few seconds, it stops.  At the least, it seems very difficult to let that sound continue for longer than say the 'ain, which is a letter of tawassut.  JazakumAllahu khairan.

Wa alaikum assalaam wa rahmtaullahi wa barakatuh
The letter , when pronounced properly does have running of the sound which is most prevalent when it is saakinah.   Its timing when saakinah equals the timing of the other letters of rikhaawah.  It is true that some of the letters of rikhaawah could  be lengthened longer if one were to continue running the sound longer than what is allowed in the Qur'an, but we have to take the timings from those who have mastered the Qur'an and have passed the knowledge down to us from the original source.  In the recitation of the Qu'ran however, all the saakin letters of rikhaawah should have equal length. 
 The  on the other hand, if pronounced from its correct articulation point cannot run long but is not completed imprisoned.   It cannot be lengthened as long as the letters of rikhaawah, if it is pronounced correctly. 
Wa iyyaakum.
Wa assalaam alaikum wa rahmatullah.