this isnt by the book,,,, just my experience,
when fallback is implemented it is usually at a temporary basis (a few hours) when fire or emergency activity is high in a boro
example - 2+ all hands at once on staten island before relocators can get proper coverage, a bunch of fires or a few multiples in another boro at the same time. it is usually by a boro to boro basis. once this freeze is over and all the pipes start breaking im sure you will hear it alot in the next few days
basically what it does is send single engine or 1&1 responses to what would usually be 3&2 responses. once a second source is received then the response will be pushed up to the normal response with a special call. but when a fall back is in place you will hear many runs for fire, smoke etc just be an engine and truck response, sometimes with a chief......
again, this is just an observed summary, im sure the books have it written alot clearer