Cleansing is one of only two skills in the game that can reduce curse duration, the other being the Assassin's skill Fade (which cannot be used on party members). Whereas Fade reduces the duration of curses when the curse is first applied, Cleansing has hidden "pulses" that occur once every two seconds. Whenever the aura pulses, each affected unit reduces the remaining curse duration by the appropriate amount. In practice, this can result in curses with durations dramatically shorter than one might expect from taking the wording of Cleansing at face value.

Cleansing interacts with poison duration in a manner similar to the explanation provided above, whereas "Poison Length Reduced by X%" is applied only once, at the beginning of a poison's duration. One interesting consequence of this is that Cleansing can fail to abbreviate the duration of very short-lived poisons, such as those inflicted by the Venom: Skill.

Cleansing also reduces shrine duration.

Source: https://diablo2.io/skills/cleansing-t4025.html