Ask HN: Meaning of initial decision parameter in rasterization?

I am talking about initial decision parameter in rasterization algorithm for lines specially.

For slope between -1 and +1, we get p0=2dy-dx

But I am unable to find how it was derived and neither do I understand what it means.

It should mean something on the line of "where to put the first pixel"....However, we just put y=mx+b and voila we get that as shown here.

https://www.youtube.com/watch?v=vvTMfJY0GCg

which is too nonsense for me to digest.