Accelerometer - Tilt Detection
It can't be much clearer than this...


  The transient function (with HPF enabled) is different from 
  the motion detection embedded function in MMA8451, 2, 3Q. 
  
  Refer to Freescale application note AN4070 for details on the motion
   detection. The intended use of these functions is different.
  
  A.  The embedded motion function triggers an interrupt when an
  y of the enabled axes has exceeded a set acceleration 
  threshold.
  
  •   Motion detection is useful when the device may be tilted
   in an orientation and the interest is to know if the 
  acceleration in one axis has crossed a specified threshold amount measured from 0g. The motion detection 
  analyzes change in threshold considering both static and dynamic acceleration.
  
  B.  The transient detection embedded function compares whether any of the enabled axes has exceeded a set change 
  in acceleration disregarding the static acceleration. The data that is compared to the threshold to be exceeded is 
  high-pass filtered data.
  
  •   Transient detection is useful when the device may be tilted in an orientation (i.e., there is some non-zero reading 
  on one or more of X, Y and Z) and the interest is more 
  on the dynamic acceleration crossing a specified threshold 
  amount in a short amount of time (frequency being the reciprocal of time) regardless of what the current value is.