At the core, they work the same as other spells in Shadowitz: force of the spell is the base damage, with yang dice adding to this and yin dice resisting drain. So far, so good.
Where we get into trouble is how these spells work in Shadowrun. Unlike elemental manipulation spells, which produce gouts of flame, acid, electricity, etc., mana and power spells automatically bypass armor. Furthermore, while a target resists power spells with his Body as any other damage resistance test, he resists mana spells with Willpower instead. Thus, a well stocked combat mage knows powerbolt for elf, and manabolt for the troll.
I've been wrestling with this for a long time. Without a damage resistance test, how do I properly represent the resilience to these spells. I worked out a number of solutions:
- Knack = threshold. In this scenario, a target's knack rating in Endurance or Resolve (the closest analog of Body and Willpower I've got, respectively) becomes the base success threshold for these spells. This means that all targets would have a base of 2 instead of 1, and it could theoretically mean you'd need 10 successes to affect a resilient target, should his knack be that high. But a 10 score is high, and I mean high. Still, the more I thought about it, the less I liked it.
- Knack = casting penalty. If you've been with me this far, you know I never considered reducing a mage's sorcery pool by his target's knack. Losing dice is not the Shadowitz way. But converting a number of dice equal to the target's appropriate knack seemed like a possible way of representing his innate resistance to the magic. Die penalties make things harder, but far from impossible. I return to the problem of scenario #1 however: a high defensive knack has the potential to overwhelm these spells. I don't like that. Spells pay in drain, while guns do not, and targets don't inflict penalties on shooters because of any ability they have. Moving on.
- Step = threshold. This is almost the same as scenario #1 above, except that the numbers are a lot lower. Now, it's the target's step rating in a knack that determines the success threshold. Thus even if a target has a Resolve of 10, the threshold to hit him with a mana spell is 5. That ups the difficulty without making it crazy. However, it requires making a calculation for every target, and even if it's a small one, it can slow things down if you do it enough. So, still not loving it.
- Knack = damage mitigation. When I hit this point I knew I was reaching. High Endurance and Resolve knacks already grant additional damage boxes, so what was I thinking here? I was thinking that Resolve grants bonus stun boxes, but mana spells, which are resisted by Resolve, deal physical. So what if mana spells with a force less than a target's Resolve knack do stun instead. Or... my mind shot off into some half formed idea at this point, but I shut down that line hard. This was getting so complicated that it was rediculous. Which brought me to the option I settled on.
- Screw it. That's right, screw it. Does there need to be some special resistance represented in these spell mechanics? Why? For bullets, you have to fold a defensive knack into your pool, and that's that. Why not do the same for these? You want to resist a manabolt spell? Fold Resolve into your die pool and use the resulting yin dice to resist. In the end, I decided there's no real reason to do anything special with these spells at all. Making them work like every other spell, weapon, etc. in the game might rob them of a little bit of identity, but makes using them much easier and makes the game easier to learn and play. In the end, that's a bigger win.