This would be for any widget types other than widgets with preset/static values (such as checkbox/button/combobox/etc). I'm mostly considering this for "slider" widgets, but I honestly didn't consider all options yet.
snapback() would be a way to tell cabbage that when mouse input ceases on a widget, it should return to the widget's default value. This behavior essentially mimics the common "pitchwheel" style behavior, but still allows for an arbitrary (non-center/assymetric) value to return to via the default set in the widget's range().
For example, this widget should return to 50 when released:
And this widget should return to 0:
I used a commercial plugin that uses a widget in this fashion, and it seemed particularly useful in their specific use case... It had essentially an rslider snapped to 0, any negative values would pull an LPF down from 20k or the ceiling, and positive values pull a HPF up from 20hz or the floor... releasing the knob returns it to 0, aka no filtering).