Apple Logic Pro 9 - Manual (Page 734)

Exponential Velocity
Alters the scaling of the velocity curve. This preset is of particular use on software or MIDI
instrument parts. Velocity is generally used to control the initial volume of instruments,
but is often used to trigger filter envelopes, making the sound brighter or darker.
Transforming regions is much faster than changing the velocity scaling or filter parameters
of your MIDI or software instruments.
Change the Velocity value field in the Operations area to change the velocity curve. This
is instantly reflected in the map area below.
734 Chapter 26 Editing MIDI Events in the Transform Window