I've set $t to control how folded the net is. The animation then previews the net as it changes from fully open to fully closed. In addition to generating animated gifs, this also allows me, with fps set to 0, to view any intermediate position by setting the value of $t. This is very handy but it would be even better to have a slider here so the value can be changed more easily.
More generally, it would be nice to be able to add more sliders which could be used to vary any parameter in a model.