Seat races are a necessary part of finding out which combination of rowers makes the boat go fastest, but nobody really likes them. Practices which include seat races are inevitably tense practices, and tension does not make for good rowing. As a coxswain, you have to be at the top of your form to cox seat races fairly. The coach needs to be confident that the information gleaned from the process is accurate. As coxswain, you have to guarantee absolute fairness. To the rowers being seat raced, this is potentially the most important race of their careers.
You must keep the ratings the same over each piece; you must steer absolutely straight; you must keep the boat set, and you must keep your calls consistent, and maintain the same pitch of intensity. Not only must you be completely impartial, but everyone on the crew must perceive you to be so. Your goal in this is the same as the coach's -- you want the fastest lineup in the boat. Ideally, no one should know who is going to be seatraced against whom. Everybody in the boat should be pulling hardest for each piece, not knowing who will be switched. (Chances are, however, that people will have a pretty good idea, especially as the season wears on.)
Whether it is one boat being raced against the clock over a certain distance, or two boats racing against each other, the results are determined by the relative difference in performance before and after switching rowers.
"...since none of the rowers knows in advance who is due to be swapped over, you can get a pretty fair reading of a decline or increase in performance...from then on it would be dangerous for anyone to slacken off, because they might be next to change."
-- Dan Topolski, True Blue
The seat race will be part of a number of factors influencing the coach's choice of lineup. It may even be part of a larger rearranging of the lineup involving more than just one or two seats. It may also be the final confirmation of evaluations which the coach has been making on the basis of many observations during the season. As coxswain, the coach will very likely consult you as well as the other rowers as to which lineup made the boat move best. You must be not only fair in your response, but be able to give objective details as to what the differences were in each piece. (Take written notes of the specifics if you think that will help.) That is also the time to tell the coach -- be fair -- if there were any variables such as ratings or (hopefully not!) steering errors, which might need to be factored into consideration. Coxswains get seat raced too. Every piece you do with your crew is a form of seat race, so put your best into each practice. Even if you are not formally switched between boats with another coxswain, coaches develop a sense of which coxswains make their boats move best. "But" -- you say -- "the rowers can influence that decision either by pulling harder or by slacking off for particular coxswains." Yes, they can, and it is their prerogative to vote with their oars. They can sense which coxswain will make them move best. Sometimes this is a matter of a particular coxswain's style suiting the style of a particular boat. The best coxswain for one boat may not be the best for another (e.g. heavyweights and lightweights are often very different in style -- more on this in a future column.) A boat might have to trade off between a coxswain with fiery intensity but who does not steer as straight, and a coxswain who steers straight but lacks the fire. Others may prefer the calm, confident type who can keep them from getting frantic. Match your style to the boat. In some programs, seniority and experience determine the coxswain order.
In the end, everyone -- coaches, rowers, coxswains -- all have the same goal in seat racing: putting the fastest possible boat on the water.