Amazing calculation technique of Indian kids:
It is the advanced level of abacus calculation, whereby the physical abacus is replaced by a virtual abacus in the mind — see the kids need to move their fingers in the air, as if they touch a real abacus. The display of the workings (moving up and down of the abacus beans) in the mind is like seeing the abacus image from a computer screen. It means also the kids are still not mentally calculating per se, they just continue to calculate on the ‘virtual’ abacus.
This is different from a real mental calculation sans a physical electronic calculator, in which case no fingers playing the pressing buttons of an electronic calculator. The brain is the real calculator, with some helps of math tricks and shortcuts.