I just read the book "Shalosh" by Nimrod Harel and learned about his trick "Add a number - BIBLE".

He wrote how to make the number you wrote in first place appear you have to put it in the memory and then when the volunteer presses the square root it appears, but it didn't happen to me.

Can someone explain me how to do that and do I need a special calculator?

The square root? The method I know uses the = key.

You can use any scientific calculator (including the one on the iPhone).