A lot of asexuals who masturbate don't do it with a sexual mindset, it's more like an exercise to relieve stress or get a little boost of dopamine.
It would be like asking "what do people without food fetishes think about when eating?" Idk, whatever is on their mind. They might think about how good it feels, but they could also be thinking about something totally unrelated.
Then why does going faster also make it easier to balance on something without steering, like a snowboard?
My assumption is that when you're standing still relative to the ground you can fall in many directions, but once you start moving momentum limits the directions you can fall to the ones in line with your motion. So the faster you're moving the fewer directions you need to worry about.