Imp defenders may release one of your jarred implings and knock your jar a few steps away during the Impetuous Impulsesminigame if you are holding an impling near one. Carrying imp repellant will lessen the chance of one releasing your impling. Their success rate is apparently dependant on your Thieving level. They are only found in Puro-Puro which makes catching the implings in Gielinor easier if your Thieving level is low.
Imp defenders always release the lowest levelled impling you are carrying. They will always shout "Be free!" when they succeed in releasing one, so this is a good way to tell if an impling has been released.
If you are hunting higher level implings, it is advised you carry around about 2 low levelled implings, so that if the imp defenders do release an impling, they only release a low level one. If the impling is higher levelled than eclectic, it will not respawn.