Do you see the dancer turning clockwise or anti-clockwise?
If clockwise, then you use more of the right side of the brain and vice versa. Most of us would see the dancer turning anti-clockwise though you can try to focus and change the direction. See if you can do it with this left brain vs right brain test.
Our conscious mind can only focus on data from one brain at a time. We can switch from one side to the other very quickly (with our corpus collosum intact) but that's not always the most efficient way to act and eventually ultimate authority to enter consciousness is delegated to one brain or the other. In our modern world, this battle is almost always won by the left brain.
It appears that most people will never reach their maximum potential because of compromises that have been made between these two governing bodies. Sometimes skills which the right brain can perform better are routinely handled, with less skill, by the left brain. Ideally, both brains work together in people with optimum mental ability. This coordinating ability may be the key to superior intellectual abilities. In most people, however, the left brain takes control, choosing logic, reasoning and details over imagination, holistic thinking and artistic talent. This explain why most of would see the dancer turning anti-clockwise for this left brain vs right brain test.
Methods have been devised to "shut off" the left brain, allowing the right side to have its say. Creative writing courses often use this method to combat "writer's block." The logical left side is easily bored by lack of input and tends to "doze off" during such activities as meditation (repeating a mantra or word over and over) or in sensory deprivation environments. The right brain is then able to "sneak" into our consciousness, filling our minds with emotional and visual vignettes and freely associated images. All too quickly, though, the left brain will assert itself and dispense with these irrational images, asserting its Spock-like logical dominance and the right brain will have to be content to find expression in dreams.