From the videos I've seen, it's essentially the same as the way you make rugs - those are sealed on the back with a wash of glue and something like hessian. Presumably you can do the same here to "lock it in", as it were.
(I did try punch needle once but managed to misread the instructions and threaded the needle wrong. Hilarity ensued.)
(I did try punch needle once but managed to misread the instructions and threaded the needle wrong. Hilarity ensued.)