What is a Stair Word™?

Question

_{This is in the spirit of the What is a Word/Phrase™ series inaugurated by JLee with his original Phrase™ and Word™ puzzles.}

If a phrase adheres to a certain rule, then I call it a Stair Word™.

Use the examples below to find the rule.

Stair Words™	Not Stair Words™
PANAMA	AFRICA
SOUPY	SOLID
WICKER	WACKER
INLET	OUTLET
WAIF	LADY
ENCHANT	IMPRESS
BORN	BARN
SIGN	SINE
QUALM	QUILT
BICKER	ARGUE
FARADAY	EINSTEIN
CALM	STORMY
FOULER	VULGAR
BANANA	ORANGE
MANAGE	GOVERN
BUCKET	PITCHER

In case you want it in CSV:

Stair Words™, Not Stair Words™
PANAMA, AFRICA
SOUPY, SOLID
WICKER, WACKER
INLET, OUTLET
WAIF, LADY
ENCHANT, IMPRESS
BORN, BARN
SIGN, SINE
QUALM, QUILT
BICKER, ARGUE
FARADAY, EINSTEIN
CALM, STORMY
FOULER, VULGAR 
BANANA, ORANGE 
MANAGE, GOVERN
BUCKET, PITCHER

The puzzle relies on the series' inbuilt assumption, that each wordcan be tested for whether it is a Stair Word™ or not on its own.

Answer 1

A Stair Word™ is a word in which

successive letter sequences (pairs) move in opposite directions on the keyboard.

For example (using an example keyboard image that I had lying around in my Pictures folder):

SOUPY and WICKER, for example, adhere to the rule:

and SOLID and WACKER, for example, do not:

as L→I→D is two consecutive moves to the left, and A→C→K is two consecutive moves to the right.  (Arguably, S→O→L is also two consecutive moves to the right, but O→L is nearly vertical.)

These are called Stair Words™ because they are suggestive

of a multi-flight staircase, zig-zagging back and forth:

         
         ^{(Image derived from http://patentimages.storage.googleapis.com/EP1181423B1/imgf0001.png)}