What is a Rotting Word™?

Question

_{This is in the spirit of the What is a Word/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.}

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If a word conforms to a special rule, I call it a Rotting Word™.
Use the examples below to find the rule.

$$ % set Title text. (spaces around the text ARE important; do not remove.) % increase Pad value only if your entries are longer than the title bar. % \def\Pad{\P{0.0}} \def\Title{\textbf{ Rotting }} % \def\S#1#2{\Space{#1}{20px}{#2px}}\def\P#1{\V{#1em}}\def\V#1{\S{#1}{9}} \def\T{\Title\textbf{Words}^{\;\!™}\Pad}\def\NT{\Pad\textbf{Not}\T\ } \smash{\lower{29px}\bbox[#ccee00]{\phantom{\rlap{rubio.2019.05.15}\S{6px}{0} \begin{array}{cc}\Pad{\T}&{\NT}\\\end{array}}}}\atop\def\V#1{\S{#1}{5}} \begin{array}{|c|c|}\hline\Pad\T&\NT\\\hline % \text{ admin}&\text{ job}\\ \hline \text{ box}&\text{ tube}\\ \hline \text{ dawn}&\text{ sleep}\\ \hline \text{ elm}&\text{ river}\\ \hline \text{ fang}&\text{ stun}\\ \hline \text{ fusion}&\text{ bomb}\\ \hline \text{ lean}&\text{ wheel}\\ \hline \text{ opal}&\text{ spots}\\ \hline \text{ open}&\text{ hotel}\\ \hline \text{ pawn}&\text{ bug}\\ \hline \hline \end{array}$$

CSV version:

Rotting Words™, Not Rotting Words™
admin, job
box, tube
dawn, sleep
elm, river
fang, stun
fusion, bomb
lean, wheel
opal, spots
open, hotel
pawn, bug

What is the special rule that Rotting Words™ conform to?

What is special about the Not Rotting Words™?

Answer 1

A Rotting Word:

Becomes another English word when rot-x is applied, where 'x' is the length of the word itself.

(NB 'rot-x' is shorthand for a Caesar shift of 'x' positions onward through the alphabet...)

Like follows:

admin -- rot-5 --> firns
box -- rot-3 --> era
dawn -- rot-4 --> hear
elm -- rot-3 --> hop
fang -- rot-4 --> jerk
fusion -- rot-6 --> layout
lean -- rot-4 --> pier
opal -- rot-4 --> step
open -- rot-4 --> stir
pawn -- rot-4 --> tear

Note that while most of these are common English words, 'firns' refers to snow that has gone through multiple thaw-and-refreeze cycles.

As for the Not Rotting Words:

All of these do still form real words when a Caesar shift is applied, but in these cases the value of the shift is not equal to the length of the word itself:

job -- rot-12 --> van
tube -- rot-14 --> hips
sleep -- rot-9 --> bunny
river --rot-9 --> arena
stun -- rot-11 --> defy
bomb -- rot-6 --> hush
wheel -- rot-7 --> dolls
spots -- rot-11 --> dazed
hotel -- rot-7 --> ovals
bug -- rot-6 --> ham