For personal medical reasons, I've had to find out more than I wanted to know about this and concluded this isn't well understood. (There may be more recent research I haven't seen.)
Some snippets of what I found:
Oral carbohydrate sensing
Studies on trained athletes holding a sports drink in the mouth for 5 – 10 seconds show performance benefits of 2 – 3 %
Believed to be unidentified carbohydrate sensors in the mouth (i.e. it doesn’t just work for sugars)
fMRI studies show an unidentified area of the brain lights up when carbohydrate is present in the mouth – that area is believed to communicate directly with muscles
Known as ‘carb rinsing’ amongst endurance athletes – when footballers, tennis players, etc, are seen spitting out liquid, it’s probably a sports drink.
Effect greatest when the body most carbohydrate depleted.
Reference
Oral carbohydrate sensing and exercise performance, 2010, Asker E Jeukendrup, Edward S Chambers
Carbohydrate in the mouth enhances activation of brain circuitry involved in motor performance and sensory perception
Clare E. Turner, Winston D. Byblow, Cathy M. Stinear, Nicholas Gant
Centre for Brain Research, University of Auckland
https://doi.org/10.1016/j.appet.2014.05.020
The presence of carbohydrate in the mouth appears to activate a novel energy signalling pathway capable of improving human performance.. This signalling phenomenon... has been shown to attenuate declines in motor function associated with fatigue.
Another reference
Carbohydrate in the mouth immediately facilitates motor output (Gant, Stinear, & Byblow, 2010)