Key in values, hitting the Enter key after each one. The operation keys (-/+, +, -, *, /) cause a calculation to be performed automatically, so you don't need to click on the Enter key for those. With an RPN calculator, you put in the values before putting in the operation. So, for example, to calculate 7 + 2, the sequence woud be '7', Enter, '2', Enter, '+'. The calculator would then calculate the result and display it via the result variable. The Backspace (back arrow) key can be used to correct the current value and the Clear key will clear out the stack and result.
I modified this to show students how to add a unary function. Take a look at the log button. The application has been modified to act more like an HP RPN calculator. It is no longer necessary to enter 2 terms to the stack prior to the operation. The value is implicitly on the stack (pushed by the binary operator). So the original instructions to calculate 7 +2 is now '7', Enter, '2', '+'. The result is set, and the value is left on the stack. To be done: button does not show motion or grey out when not available.
