What does 'twanger' mean in the next sentence from the 'Harry Potter: Order of the Phoenix', page 616
To activate the hex, just say “Plunk your magic twanger, Froggie!”
My understanding is that the sentence means something to this effect: To activate the curse, just say "Trow down your magic xxxxxx, Froggie!"
But what does the word 'twanger' mean in this context?