[randomswede]

I have always understod "positive feedback" as "feedback that prompts the existing change to continue in the same direction, with equal or larger speed".

So positive feedback on a falling signal would tend to make it drop more. And positive feedback on a rising signal would cause it to rise more.

So, basically, for any signal S at time t, we would expect something like S(t+1) = S(t) + S'(t) * k, for some k > 0. And blatantly abusing derivatives for "should probably be a delta between S(t) and S(t-1)".

But, then, I am not a control theory specialist, I don't even play one on TV.