Stable charms are called so because they always last for the duration of the nano (unless you do something like free/terminate or attack them). Unstable charms allow the mob to constantly roll to see if they break the charm - it won't last any longer than the nano duration, but they could break it at any time before. Why would you ever use unstable? Because you can only use stable if you have 0 charmed mobs (i.e. it's your first charmed mobs), but you can use unstable with 1 mob (stable or unstable) already charmed.
Could someone go over how to free (not terminate) a charm? /pet free isn't a recognized command. I would love to have a way to get around /pet "%t" terminate 's problem of releasing both charms if they have the same name.