When pronouncing kasrah, do we have to lower the chin only or the jaw as well?

Question
Assalamu Alaikum

- When pronouncing kasrah, do we have to lower the chin only or the jaw as well? I have noticed that it is possible to lower the lips (and chin) without any movement of the jaw.

- if a letter of qalqalah is sakin and the letter next to it is also sakin (as a result of doing waqf on that word), do we have to do qalqalah in this case? e.g. the word "bil-adl" in 2:282 or "qadr" in surah qadr. Do we pronounce the qalqalah on daal in the above 2 cases?

Answer
Wa alaikum assalaam wa rahmatullahi wa barakaatuh,
When we refer to lowering the jaw for a kasrah, we do not mean opening the jaw, rather more like retracting it as we do when we smile. 
The chin does move during the kasrah but as a result of the jaw movement..  If you put your finger on the temporomandibular joint by the ears (the jaw joint) you will feel the movement when you retract your jaw for the kasrah. 

Yes, we still make a qalqalah on the saakin qalqalah letter before a saakin due to a stop as in the examples provided in the question.  It takes a bit of practice to perfect it, but it is not too hard.