Traditional abacus method uses only 1-hand (right hand only) to stir the abacus. As a result, it stimulates only the left brain but not the right side of the brain.
On the other hand, because the 2-hand method uses both hands, it stimulates and train both the left and right brain for a balanced and total brain development.
It is also found that the 2-hand method is faster and more accurate in mental arithmetic calculations. Imagine texting on your mobile with just one hand.