Background count is described in the Street Grimoire (PDF p. 32). However, how it affects adepts is really vague at best:
A background count imposes a negative dice pool penalty equal to its rating for all tests linked in any way to magic (such as spellcasting, summoning, and skill tests that use active adept powers such as Killing Hands or Improved Sense).
That's fine – whenever an Adept rolls for usage of his active power (like Killing Hands I presume) there is a penalty to the dice count. However, later in text it reads:
Adepts may use a Simple Action to turn on or off a passive power in cases where penalties from background counts might exceed bonuses from their powers.
What penalties? It didn't mention this before, so I'm confused. Let's assume for a moment that this is for skill rolls that are affected by Adept's passive powers like Improved Reflexes. What about limit enhancers like Enhanced Accuracy?
In other words – how does background count affect an Adept's passive powers?
Best Answer
Background count has been toned down from 4th edition. Back then, when an adept entered a background count, their magic was offset by the amount of the background count. They had to, then turn off powers that they couldn't sustain with their new power point limit. If they had 6 points of powers and entered a background count of 1, they had to turn off enough to get it under 5.
Now, background count is simply a penalty that is applied any time you use a magical thing, be it active or passive. This means that your adept that took an Enhanced Accuracy with magic is using that to boost their limit to a weapon by the rating of the Enhanced Accuracy. If you had an Enhanced Accuracy of 2 and walked into a background count of 1, you're fine. You still get a net of 1. However, if you walk into a toxic waste dump and have a background count of 3, your mana is now screwed up to the point that you're getting a -1.
It's easier to understand it like this. You have bad eyesight. You put on a pair of glasses. The glasses are the mana that's weaved around your eyes to make them see better. Normally, with an eye-glass prescription, it's made so that you get a 'bonus' to your eyesight. However, those glasses can have all sorts of things happen to them. Fog, being broken or scratched, or even having the lenses melt and shift the image and bend of the glasses. Now, you're taking a 'penalty' to your eyesight. The game is giving you an out by saying that you can turn off a passive power so that the negative mana doesn't interfere with your normal mundane skills. Ergo, you can 'take off your glasses' until such time as you get out of the area that's fogging them up.
In cases of Improved Reflexes, I would rule that you would have a -1 to your Reflexes for every background count. The contention would be what happens to your added dice. My opinion? You'd lose a die for each point of background count, down to your minimum of 1.