Below an example:
father: 30.4.1956.
3 & 1
date - 3+0 = 3
date and year = 3+0+4+1+9+5+6=28
= 2+8 = 10
=1+0=1
so 3 & 1 for father
mother: 20.10.1961
2 &2
date = 2+0=2
date and year =2+0+1+0+1+9+6+1
=20
=2+0
=2
so 2 and 2 for mother
Now consider the dob of son or daughter
son dob: 28.10.1983
1& 5
date 28=2+8=10
=1+0=1
date and year=2+8+1+0+1+9+8+3
=32
=3+2
=5
so 1 & 5
Now the magic:
the father and mother has 3,1,2,2 in their dob
so now son has 1 & 5.
He has no 1 from father.
so the son dob will have atleast one of the number of father or mother. In some times the son or daughter will have both father and mother dob numbers.