It doesn't, unless you account for air friction, in which case there is a lift force that pushes up in some cases. You can think of it as changing the pressure differential that generates the lift, assuming we're talking about something like say a helicopter.
edit.. In the general case of a three dimensional object alot will depend on the geometry of the object. For instance, it might actually speed up the object (relative to another that is free fall with air friction), think spiralling football plummeting to the earth.
The eqns are rather complicated, and an exercise in fluid mechanics for the general case.. Since lots of little vortices can be created by asymetric rotational motion.