It's called inertia. Just because you turn off the engine it does not keep the wheels from spinning. Placing the shifter in park won't do much, if anything, because the transmission does not slow the car down unless you drop your automatic transmission into the 2 or 1 setting. But if you're forced to remember all this in two split seconds.............

Downshifting also helps to slow your manual transmission down too. Screw damaging your clutch or transmission, just make the thing stop.

And to follow up on wolverine, keep a back up plan, like downshifting [[into 2 or 1) if your emergency brake fails. Since there are very few hills around here where it's necessary to apply the "parking" or emergency brake [[where the usage alone keeps them operational), they do rust and get stuck, and may or may not work.