what are masked assignments

I came across following code while going through intel's  Auto vectorization tutorial,

for (int i=0; i<length; i++) {
float s = b[i]*b[i] - 4*a[i]*c[i];
if ( s >= 0 ) {
s = sqrt(s) ;
x2[i] = (-b[i]+s)/(2.*a[i]);
x1[i] = (-b[i]-s)/(2.*a[i]);
}
else {
x2[i] = 0.;
x1[i] = 0.;
}

“if” statements are allowed if they can be implemented as masked assignments, which is usually the case.
so how exactly does the masked assignments work ? I think i have a little grasp on the masked assignment concept from here.
are these bit vectors are calculated via compilers by applying some heuristics or at run time ( BPU) ?

i understand that bit vectors with TRUE value are clubbed together and viceversa for FALSE & both of them are executed altogether.
but how exactly the mechanism works with intel machine ?

also what will be the scenario if code is non-straight line;
i.e.
 

for(i=0;i<lengthA;i++)
 for(j=0;j<lengthB;j++)
   {
 float s = a[i][j]*b[j][i];
if ( s >= 0 ) {
c[i][j] = sqrt(s) ;
}
else {

c[i][j] = 0.01 ;

}
}

A simplified/tutorialized explanation will be very fruitful .Eagerly awaiting your reply ,