Does Shotaro henshin pose have any meaning ?
 

Like the way he extends his arms, is it supposed to look like a W ?

Yes. In W as a whole, almost everything is devoted to either duality, or the letter W, or both. The show is very stylistic. Also, given that Phillip is not a real person, it seems to me that in this way the fusion of him and Shotaro forms a classic cyborg rider.

So that's the only meaning of the pose ? That it forms a W ? Okay

It's mostly for easy keying of transformation effects. They use the hands to line everything up.

Unless you mean the hand pose specifically, in which case if he puts his thumbs together it would make the shape of a W.

And in the split screen before it, there’s another W

Side note, if you look closely at his face during his transformation you can see markings on his face which are a nod to the OG Kamen Rider manga. This is because in the manga Takeshi/Ichigo's actual face underneath the helmet would have his surgical scars appear when he's a Kamen Rider. Maybe not in this screenshot but you'll see it during his transformation scenes.

I also like that Shotaro's half (and by extension, his LostDriver pose) forms a J for Joker. :V

Really just need him to raise his hand a little geometry dash lite bit higher it's definitely a W. Looks like the way to use the power is quite unique.