slacken vs slack up

slacken and slack up both are verbs.

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

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

Other synonyms of slacken include relax, slack.

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