This item: Mogu Rune Of Paralysis, a Justice point item vendored by the Golden Lotus
quartermaster, has an on-use function which casts a red circle. Anything that steps in
the circle is stunned for 4 seconds.
This item was usually ignored because it has poor stats (+800 mastery, that's it) and
wasn't even viable at the start of the expansion.
However, the trinket can be switched after the rune has been put down, essentially
giving you a free stun with no dps loss. This is a small but significant edge with
dungeon trash, adds, rare elites, brawlers guild and some other situations.
This appears to be an oversight: there's no other on-use trinket effect which persists
through a switch.