What is Least-Constraining-Value?
see this link: 

https://people.cs.pitt.edu/~wiebe/courses/CS2710/lectures/constraintSat.example.txt

It first picks variable "O" and then tests "O" with all of it's legal values "i" to see the number of reductions on "O"'s neighbors "N". It adds all of them. and picks an "i" that causes less reductions:

   sums = {0:0,1:0,2:0,3:0,4:0,5:0,6:0,7:0,8:0,9:0}
   For i from 0 to 9:  
     plug "o=i" into the constraint formulas
     For each neighbor "N" of "o" in the constraint graph:
       sums[i] += the number of values remaining for "N"


It picks "i" so that: 

sums[i] = MAX{sums[i] | for all "i" that is a member of "O",s valid values}


I hope This can help you find your answer!