At the risk of sounding anally retentive (what has four years of undergrad aero eng done to me!).......
The net force generated by a body in flight cannot be forwards - that is, into the flow. However, we commonly resolve the total aerodynamic force on an aerofoil (which a winglet is) into components parallel and normal ('drag' and 'lift') to the chord line of the aerofoil. By appropriate angling of the section it is therefore possible to make some of the 'lift' appear to be 'thrust' (i.e. angled into the flow), although the 'drag' generated will be larger than this component, ensuring the total force generated is not forwards.
Sounds a bit complicated - would be helped by a diagram I guess! The concept is similar in character to the way the lift force generated by the wing of a glider is angled forwards to oppose aerodynamic drag, in addition to the aircraft's weight in unaccelerated (but decending - at least relative to the airmass) flight.
Sorry if I've confused anyone more!