I think the reasoning goes because you are used to using carbs as your energy source most of the time, your body doesn't know what to do at first when it doesn't have any available. The body adapts surprisingly well to different energy sources, so if you drop all carbs, your body will burn fat for energy and create ketones.

It kinda sounds crazy, and I used to think it was, but I'm currently low carb to lose weight (lost 28 pounds in 3 months with it so far!). It just takes a couple weeks of training while on the diet to get your body to catch on and be back to your usual performance.

There are actually a surprising number of world records set by athletes on low carb diets who destroyed previous records that claim their diet is the main reason they were able to accomplish their goals