Well I'm not 100% sure, but it looks like that is dependent on the player's heading. So it doesn't push in the right angle, unless the player is facing exactly the right direction (i.e., square centered on the thing pushing them.) It just sends them backwards from whichever way they are currently facing, I assume. Which isn't what you want at all, really.
You might be able to get that packet to do what you want by changing the "spu->heading" to be the inverse of the caster's heading (or this->CalculateHeadingToTarget(caster->GetX(), caster->GetY()) I should say), rather than the player's current heading. Or maybe play with the "spu->delta_heading" field? dunno what that's for... actually, nevermind, that would probably just spin the client around. You wouldn't be able to rely on that packet (at least not alone) for NPCs though since it doesn't actually change their serverside position.
(Vectors and trig are equivalent in this case I guess, but I hate trig even more ;p)
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