loosening vs slackening

loosening and slackening both are nouns.

Word	Noun	Adjective	Verb	Adverb
loosening	Yes	No	No	No
slackening	Yes	No	No	No

As nouns, loosening and slackening are synonyms defined as:

loosening and slackening: an occurrence of control or strength weakening

Other synonyms of loosening include relaxation.

loosening (noun)	slackening (noun)
the act of making something less tight	an occurrence of control or strength weakening
an occurrence of control or strength weakening