This glitch occurs when a unit is under the effects of a status condition and they are then rescued by another unit. In this context, a Status Condition can be something harmful like poison, or like the positive effects obtained from Ninian and Nils' rings. When a unit is rescued the rescued units status condition does not wear off and the turn counter for it to wear off does not go down.
This glitch is often used by players in The Blazing Blade with the Dancer rings, Ninis' Grace, Thor's Ire, Set's Litany, and Filla's Might, to abuse the Arena and save uses on the rings for later use. Ninis' grace is the most often used, as it gives a defense boost to a unit which the arena does not calculate, often leading to matches where the enemy in the arena does no or minimal damage.
The glitch can also be used to prevent a unit from taking damage or dying from poison.