Greek researchers have been found that swallowing a small amount of water can start the fight against dehydration ... So, in subjects dehydrated, drinking 25 ml of water during exercise tends to increase performance.
They  dehydrated sportsmen (2 %) by the realization of moderate exercise (running and cycling) in the heat. Following this protocol, they realize a cycle ergometer test to exhaustion at 75% of their maximum power.


When the athletes were drinking tiny amounts of water (25 ml) every 5 minutes, performance time tended to be improved. However, this increase was not significant.

As the quantities consumed are very small, the Greek researchers have speculated that activation of pharyngeal receptors could enhance performance

Bibliography:
Small Amount of Water Ingestion but not Mouth Rinse Improves Exercise Performance in Dehydrated Athletes. Medicine & Science in Sports & Exercise: May 2010 - Volume 42 - Issue 5 - p 112