While? Do people do that? My father would have kicked my butt for wasting water if I ran it during rather than just rinsing the brush and sink when I finished. But then he also installed a water valve in the showerhead so you could turn off the water while sudsing up. That rarely got used.
I was thinking of the water you take in and spit out, not keeping the tap running. I didn’t word it right.
But think of all the millions of mouthfuls of water wasted just wetting your teeth at the start and rinsing at the end. Horrible. Save the earth. Stop brushing!