I have 2 questions asked by students of Tajweed studying with me the Riwaayah of Hafs ' An 'Aasim min tareeq Ash-Shaatibiyyah.

Question
Assalaam alaikum wa rahmatullahi wa barakaatuh
I have 2 questions asked by students of Tajweed studying with me the Riwaayah of Hafs 'an 'Aasim min tareeq Ash-Shaatibiyyah.  The first is concerning how to stop and how to continue on the word < ولِيِّ in surah Yusuf, aayah 101.

The second is concerning the saktah: What is the reason for it? Is it simply by reason of how it was passed down or is there another reason as well? Jazakumu Allahu bi kulli khair!
Answers
Wa alaikum assalaam wa rahmatullalhi wa barakaatuh
First Answer
The word < ولِيِّ has a ya' with a shaddah and a kasrah followed by an unwritten but established ya' saakinah.  This small ya' is read when continuing and when stopping.  So we have a with a kasrah (the second part of the shaddah) followed by a  ya' saakinah.  We read the first with a shaddah and a nabr then read the natural two count medd which occurs with the second saakinah.
Second Answer
There is no reason for the four saktaat of Hafs other than it is the way which was passed down to us from the mouth of our beloved Prophet, peace and blessings of Allah upon him in a sound chain for the riwaayah of Hafs 'an Aasim min tareeq Ash-Shaatibiyyah.  No other qiraa'ah or riwaayah has these four specific saktaat.  They are widely known as saktaat Hafs. 
Wa iyyakum wa-l-muslimeen