Recently we have been doing some long, moderate intensity sets where we ask you to maintain a HR in the 130 - 140 area. From an energy system training standpoint this type of set improves your body's ability to produce energy for work through lipid metabolism - burning fat. This moderate intensity stuff is low stress work that you can carry on for extended periods of time. And, truth be known, this can get boring if you let it. As you know, speed is not always directly related to effort.
Speed is more directly related to efficiency than energy expenditure.
This gives us a natural, and very challenging, focal point for moderate intensity sets. The idea would be to see how fast you could swim each repeat while staying in the HR range prescribed. Not working harder, just swimming faster. The key here is to improve the efficiency of the swim. Decrease your SPL by increasing distance per stroke, improving your streamline position, longer glides off each wall, being aware of and avoiding unnecessary resistance etc.