Negative split swimming strikes some as counterintuitive. It doesn't feel right to hold back effort at any point in a competition, least of all at the outset. Indeed, negative split swimming won't lead to faster times until after significant training. However, many coaches maintain that once a swimmer finally does master negative splitting, it'll result in greater control throughout.
First, it's important to understand why swimmers can't keep up their full force throughout an entire race. Your best speeds recruit anaerobic metabolism efforts, which use carbohydrates and not oxygen, causing your body to produce excessive amounts of lactic acid. Coach Gale Bernhardt explains that this lactic acid builds up in tissues and soon prevents enzymes from doing their job of breaking down carbohydrates effectively. Other problems include the damage to cell membranes and a buildup of electrolytes inside cells, which leads to cell swelling [source: Bernhardt].
In addition to these and other physiological factors that can drastically inhibit performance after a burst of full-force swimming, you simply waste energy inefficiently this way.
All this is to say what all athletes know intuitively -- that you can't keep up high intensity for long. Pacing, therefore, is the best way to go for longer races. Ideally, you can learn to recruit that burst of energy for the final stretch of a competition while still taking as much advantage of that burst as possible.
Esteemed coach Mark Schubert swears by negative splitting, claiming that even though the technique isn't for everyone in competition, the skill is invaluable to learn. To avoid the disappointment of being taken from behind, a swimmer who trains in negative splitting will be able to maintain a strong end to a race. But even those who do excel at negative splitting shouldn't always use it, argues Schubert. Instead, it's best to vary your technique and stay unpredictable for your competitors [source: Schubert].
But negative splitting is as much mental as it is physical. Those swimmers who start off too strong can get easily frustrated when losing energy and being overtaken on the second half. They experience slower speeds with what seems like greater efforts. In this sense, negative splitting teaches patience, confidence and control.