The aim of خَزَنَ لِسَانَهُ is to restrain lies, gossip, humiliation, slander, and needless injury. It does not imply remaining silent in every instance of injustice or concealing testimony that is needed to protect someone. Truthful, measured speech can itself be a duty.
Example 1: Privately help a colleague rather than exposing an embarrassing weakness at a gathering.
Example 2: Refuse to forward an unverified accusation.
Example 3: Provide reliable information when it is necessary to support a person who has been wronged.
Protecting the tongue means avoiding wrongful speech while communicating necessary truth with wisdom.