If you swing to the right, step to the right, vice versa.
The hard bit is timing it to your opponents movements, waiting for them to step one way, you step and swing the otherway.
Actually, when I tested the speed bonus in SP (I guess it works the same way in c-rpg?), it appeared to me that:
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login1) You get more speed bonus by simply moving straight towards your target than moving to the side, even with a side swing.
2) You get even more speed bonus if you move forward but turn to the side as it shown in the pic. Note that you don't sidestep with A or D (you would lose speed that way), but just turn your mouse in your swings while keep pressing W.
So the third case is the best if we talk about damage alone, IMO.
PS: all this is implying the enemy stands right above the "character", was to lazy to draw that.