I thought it was the advice that by the time you are thirsty you are already dehydrated?
No that was the dogma invented by the likes of Gatorade, Camelback and the likes. One of them has the slogan, "Hydrate or die." They also promoted the isotonic nonsense, which was just corn syrup they could not get rid of other than to invent a new category of drink that was supposed to replace water.
If you are thirsty, you drink. It is silly to say that if you are thirsty, you are dehydrated. That means my dog is dehydrated all the time, not to mention you and I. Most endurance athletes perform better if they are slightly dehydrated, especially less trained ones. They benefit from the one or two kgs they loose in water and suffer no ill consequences from it. By over hydrating on the other hand, you die. Up until Tim Noakes presented his papers on this topic, one or two marathon runners died at each major marathon (hot and cold days). Now doctors are trained to look out for water logged athletes and treat them with....wait for it. A massive dose of salt IIRC.
Once you are sensitized to what's going on, you'll see the hydrate or die mantra jump out at you in all sorts of unusual places where the only beneficiary is those who peddle sugary drinks and ultimately diabetes. Diabetes amongst elite endurance athletes is alarmingly high, because of this hydrate or die nonsense.
But if you are interested, why not read up on Tim Noakes? I've provided one link, but I'm sure it'll lead you to other useful links and eventually on a path that resembles the tree of life that will make a turn somewhere at Indian Pole Dancing and a wet T-shirt competition in Omaha.