Hi peter_l_downs - the summarizer is based on Open Text Summarizer (http://libots.sourceforge.net/) which works very similarly to your page at bookshrink (TF-IDF sentence scoring). I made some minor edits that accommodate article structure.

Bookshrink has some pretty amusing summaries... it reminded me of this meme a while back where people would paste books into Microsoft Word and AutoSummarize it down to 6 words :)