understanding about idea in a Matlab Graphics blog

Hi:

I read through a Matlab blog : https://blogs.mathworks.com/graphics/2015/07/22/implicit-surface-intersections/

I'm able to understand the detail process until a post here:

Here's where things get a little messy. Each row of out_vert contains one 0 and two 1's. We want to draw a line between the edges of the triangles which connect the vertex with the 0 with each of the two vertices with a 1.

and the solution is command shown below:

ntri = size(out_vert,1);
overt = zeros(ntri,3);
for i=1:ntri
    v1i = find(~out_vert(i,:));
    v2i = 1 + mod(v1i,3);
    v3i = 1 + mod(v1i+1,3);
    overt(i,:) = crossing_tris(i,[v1i v2i v3i]);
end

I'm confused about the idea and command, so it is hard for me to move forward.

could anyone give me some explannations about it?

Thanks!

Yu