Depends on how the effect is described in flavor terms. My impression is that its something like a Displacer Beast so they get like all wavy as they are moving, thus the lack of penalty if not moving at least 3 spaces.

Yes, he isnt invisible with the shadow walk.Just walk betwen the shadow plane and the real plane at the same time . Its more dificult see teh warlock but you can atack him with a -2 penalty check to hit. Other thing is than he use the Will power eyebite making to him invisible against the enemy he use the power. But ONLY against this foe until the star of his next turn.And...he can give oportunity atacks to his foes when move. Because is visible and walk. He dont teleport.Sorry for my english . I hope this will be usefull to you.


Edit: I think when a warlok walk in shadows his figure fades away, being wrapped in darkness on having be mixed to his step between both planes of existence.

Think of it this way.

When he moves he is surronded by shadows. Much like the Balrog from the Fellowship of the Ring movie was surronded by smoke.

You know he is there from near or far away. You just aren't sure EXACTLY where his arm/leg or hand is at any point in time.

Another way to think of this is... dim light (candle light for example) or heavy fog makes it so you know someone is there but it's hard to find see exactly the detail of what they are wearing. As such it's hard to know exactly where the gaps in his armor is that you are supposed to aim for.