In the spirit of the What is a Word™/Phrase™ series started by JLee, a special brand of Phrase™ and Word™ puzzles.
If a word conforms to a special rule, I call it an Bi-Directional Narcissistic Word™. Use the following examples below to find the rule.
$$\begin{array}{|c|c|} \hline \textbf{Bi-Directional Narcissistic Words™}&\textbf{Not Bi-Directional Narcissistic Words™}\\ \hline \text{MOTTO}&\text{GOAL}\\ \hline \text{AIM}&\text{TARGET}\\ \hline \text{OVUM}&\text{EGG}\\ \hline \text{TWO}&\text{ONE}\\ \hline \text{THAT}&\text{THIS}\\ \hline \text{WHO}&\text{WHICH}\\ \hline \text{WHAT}&\text{WHEN}\\ \hline \text{WHY}&\text{WHERE}\\ \hline \end{array} $$
And, if you want to analyze, here is a CSV version:
Bi-Directional Narcissistic Words™, Not Bi-Directional Narcissistic Words™
MOTTO, GOAL
AIM, TARGET
OVUM, EGG
TWO, ONE
THAT, THIS
WHO, WHICH
WHAT, WHEN
WHY, WHERE
The puzzle satisfies the series' inbuilt assumption, that each word can be tested for whether it is an Bi-Directional Narcissistic Word™ without relying on the other words. These are not the only examples of Bi-Directional Narcissistic Words™, more, but a very limited number exist.
What is the special rule these words conform to?