Purge

A Purge is a combination of basic dispel and movement speed (and sometimes also attack speed) slow. In some cases, they also apply root, preventing units from moving and using certain abilities for a short duration.

Mechanics
The dispel purges apply is a basic dispel and it is only applied once upon cast. It is not applied continuously.

The slow of a purge works different than a regular slow. All purges (except for ) initially slow a unit's movement speed down by 100% (and attack speed by 100, if the source of the Purge slows attack speed). The slow gets gradually weaker over the Purge's duration at a certain rate. The rate and amount at which the purge gets weaker is determined by the update frequency. The amount of instances is equal to the update frequency, while the amount of slow loss equals the initial slow amount divided by the update frequency.

If a purge has an update frequency of 1, it slows for the same amount for its whole duration. If it has a frequency of 2, it slows for 100% of the starting slow value for half its duration, and then for 50% for the remaining duration. With an update frequency of 3, it slows for 100% for the first third of its duration, then for 66.67% for the next third of the duration, and finally for 33.33% for the remaining duration. This means the higher the update frequency is, the smoother the intervals and slow loss get.