Shedding orbital velocity would push it outwards away from the sun. What it basically did was do that relative to the ballistic path that the initial launch (as a passenger with Akatsuki) had put it on, thus altering her path distinctly. However it didn't just do that.
That B-plane diagram above on the right (with the "projected points") shows where Ikaros would fly by Venus if it stopped adjusting its path on a certain day. If you don't steer the sail then the pressure on the sail would mean that this flyby point would constantly move "outwards" relative to Venus, on the above diagram that means it'd move from left to right.
For the first six weeks they angled and steered the sail in such a way that the projected flyby point instead went "inwards" - i.e. they sailed angled against the wind, until they reached a point where Ikaros' path wouldn't take her by Venus "behind" her but instead "in front" of her relative to the sun. End of september they then dropped this steering and as expected the wind pushed the craft "outwards".
Edit: This might explain it better in 2D, as a quick-and-dirty MS Paint picture:
The black line is your path that you were pushed on. Since you have a sail, the solar wind (here coming from the left) pushes you progressively further off, following the red line. What Ikaros did was turn into the wind in such a way that they followed the green line.