It took me a while to figure this out, but when I realized that these could be re-written as:

  x = alpha * x + (1-alpha) * this._lastMotionData.x

then I understood that this is just an exponential moving average (see http://en.wikipedia.org/wiki/Exponential_smoothing#The_exponential_moving_average.)  You might use that phrase in a comment here to make it clearer what kind of filtering is being done here.

The way the formulas are written here they only require one multiplication, so this way is better.