The listing, Wilson Hollow Core has ended.
I don't know much about since they sent to me free with some magazine!It is to re-string a tennis racket, It is made by Wilson & says hollow core 16. Never used!
Found this:One of the most unique strings on the market, Wilson Hollow Core 16 offers a combination of power, comfort and control. Featuring a triangular, air filled core, Wilson can reduce the amount of polyurethane resin used in binding the multifilament fibers. Hollow Core uses three layers larger fibers typically found in outerwraps on top of this air filled core. By using larger fibers instead of traditional multiflament fibers, the overall response is similar to a multifilament, but with increased durability and without the fraying commonly seen in multifilament strings. This string is best suited to players seeking a very comfortable, control string that provides a little extra power.