If you have either the TOEP or the Zandalar charm equipped, but not both, this will switch them around after you've used them (if you're out of combat and the cooldown is still above 30 seconds ; 30s being the default cooldown when you equip a trinket)
This essentially saves a trinket slot for something else like Neltharion's tear
The trinket gets equipped after the buff has faded out, as soon as items can be equiped (out of combat, alive, not casting, not stunned)