A protein's distance matrix is the n by n symmetric matrix that gives the pairwise distances between a protein's amino acid residues.
This page shows the binary contact map of a given protein by truncating the distance matrix at a length specified by the slider.
Rather than condense residues to single points from which to measure pairwise distances, some use the minimum distance between any heavy atoms (not currently implemented).
The principal eigenvektor of the contact map is shown above the matrix, and can be thought of as an “importance score” for a given residue, and correlates with residue depth.
This eigenvektor is calculated is JavaScript via the power method, a simple and fast iterative procedure.
You might have better performance using a browser with a fast JavaScript engine (Google Chrome).