When you make a starter (and you should almost always make a starter) you don't need to smack the thing. All smacking the pack will do is to let you know that the yeast isn't dead. There is just enough nutrient in there to wake up the yeast and that is it.
In spite of what even the Activator packs say, there is still only about half the number of yeast cells for proper fermentation of a 5 gallon batch of beer. A 2 quart starter will more than double the amount of yeast in 18-24 hours. This will give you a proper amount of really healthy yeast. If you underpitch the yeast, you will get a slow start to fermentation which will give any competing bacteria to get a foothold before the yeast multiplies enough to crowd it out. Also if the yeast spends a long lag time reproducing, you will get excessive esters which may not be pleasant in the resulting beer. Underpitching will also stress the yeast leading to it shutting down before eating all available fermentable sugars, leaving you with an underattenuated, too sweet beer.
Bugeater Brewing Company