matlab code for face detection using RGB space color && skin color

I have this matlab code for face detection using RGB space color and it is works 100%

clc

I=imread('my.jpg');

figure,imshow(I);

%%skin region in rgb image%%%%%%%%% if(size(I, 3) > 1)

    final_image = zeros(size(I,1), size(I,2));

    for i = 1:size(I,1)
        for j = 1:size(I,2)
            R = I(i,j,1);
            G = I(i,j,2);
            B = I(i,j,3);

            if(R > 95 && G > 40 && B > 20)
                v = [R,G,B];
                if((max(v) - min(v)) > 15)
                    if(abs(R-G) > 15 && R > G && R > B)

                        %it is a skin
                        final_image(i,j) = 1;
                    end
                end
            end
        end
    end
end %%%added
    figure, imshow(final_image);
    disp('before');
BW=final_image;
%else
%    BW=im2bw(I);
 % figure, imshow(BW);
%disp('convert');
%end

L = bwlabel(BW,4);

BB = regionprops(L, 'BoundingBox');

BB1 =struct2cell(BB) BB2 = cell2mat(BB1);. [s1 s2]=size(BB2);

mx=0;

for k=3:4:s2-1

    p=BB2(1,k)*BB2(1,k+1);

    if p>mx && (BB2(1,k)/BB2(1,k+1))<1.8

        mx=p;
        j=k;
    end
end

the questions 1. why for- loop "for k=3:4:s2-1" ??

2. what does this loop ??

3. what dose "if p>mx && (BB2(1,k)/BB2(1,k+1))<1.8" ?

4. what step of loop is 3 ??