You could describe it as 'interfering with the formation
of the tip vortex and thereby reducing drag', but an
easier to understand explanation (and just as accurate)
is that of the Thrust component of the force vector
produced by the winglet. The crossflow component generated
by the tip vortex means that the angle of incidence
of the winglet relative to the flow is different to
that relative to the line of flight. If this angle
is large enough then the winglet itself will no longer
produce a force component opposing the direction of
travel, but will instead produce a small 'Thrust' component
in the direction of travel. The fact that the winglet
is in fact producing a force in the direction of travel
is why the term 'Thrust' is perfectly correct.
Its easier to explain using a diagram, and plenty of
textbooks have them if you really are that interested.
The important thing to remember is that because of
the influence of the tip vortex, the flow striking
the winglet is not travelling in the same direction
as the freestream velocity. Thats what makes it all
possible.