What does the "approx" key do in RPN mode?

[pinkman]

Shift Enter calls approx().
approx() is a CAS command. In CAS view it takes the selected or the last value on the stack and returns a numerical approximation. If you call approx() with no arguments, it returns [].
In home view (Textbook or Algebraic mode) it shows the command and you can add arguments.
In Home view RPN mode it runs the command directly, so you can’t see that no arguments are given.

If you really need to use approx() in RPN mode (but maybe you needed EVAL on the shift comma key), put your arguments on the stacks, then call approx(1) to approximate the expression on stack 1, or approx(2) to approximate the expression on stack 2 with precision given on stack 1.