Sport and SAS (surgical arthritis service which is an arthroplasty rotation) are the two most popular away rotations, probably because of the sentiments expressed in these posts. Students on the sports and SAS services will work with Dr. Williams and Dr. Sculco respectively, but this will not give you any real advantage over other applicants. While a chairman may have the theoretical ability to have the final say in choosing the residents, that ability is not really exercised at HSS. Interviews and the rank list are determined by consensus of the resident selection committee, which is comprised of about 20 attendings and the chief residents, all of whom participate in the interviews of all the applicants. So it not necessary to impress Dr. Williams or Dr. Sculco before the interview, but your goal during an away rotation is to impress everybody. More realistically, a highly successful away rotation will result in a strong endorsement from one attending, the residents you worked with, and a generally positive feeling from anyone else that you may have interacted with. Consequently it is much better to work closely with a couple of attendings rather than trying to work with as many as possible. This is especially true on the sports service, where there are more attendings than days in a month.
Here are the primary people rotators work with on other services, all of whom are excellent and love to have students:
Trauma rotation: Dr. Helfet and Dr. Cornell
Spine service: Dr. Oleary and Dr. Cammisa
Hand service: Dr. Weiland, Dr. Wolfe
Peds service: Dr. Widmann and Dr. Green
Foot service: Dr. Deland and Dr. Roberts
Metabolic bone: Dr. Lane
Fracture: Dr. Lorich