slack up vs slacken

slack up and slacken both are verbs.

Word	Noun	Adjective	Verb	Adverb
slack up	No	No	Yes	No
slacken	No	No	Yes	No

As verbs, slack up and slacken are synonyms defined as:

Other synonyms of slack up include relax, slack.

slack up (verb)	slacken (verb)
make less active or fast	become looser or slack
	make slack as by lessening tension or firmness
	become slow or slower
	make less active or fast