Some people were very good at it, others horrible.
One revealed the method they were using - they didn't count audibly, they visualized a ticker tape moving across their vision with numbers increasing. Or say a rotating scale with the numbers rotating. This let them read or internal monologue as the senses are now separate.
I tried to practice for a bit, still impossible to do without thinking about it. Kind of like how people default to counting money in their native way.