The slack rope needed to be tightened to make it more secure.

需要把松弛的绳索系紧，这样会更安全。

youdao

I shouted as I broke my jump rope rhythm and tangled my feet in the slack rope.

我大声地尖叫着，打乱了跳绳的节奏，脚也被松下来的绳子缠住了。

youdao

The rope suddenly went slack.

绳子突然松了。

《牛津词典》

At the top of each pitch, he pulled up the slack in the rope, fixed it to the anchor and started climbing again, while I used mechanical ascenders to get to the station and put him on belay as fast as I could.

WSJ: El Capitan's Nose in a Day | By Michael J. Ybarra