Ill give it a go.

Let's say that your destination TAF doesn't look so hot. You need 1/2sm to go there and the TAF says tempo 1/4. 3585 allows you to still dispatch there, but requires a second alternate to be listed, and fuel for the furthest alternate. The caveat is that the weather below your minimums must be at least 50% of what you need, and must be part of a conditional statement in the TAF (part of a prob30, prob40 or tempo line). Same can be done if your first alt is below the derived alternate mins, same rules apply ( conditional and 50%). 2nd alternate has to meet all weather mins.

That's about as simple as I can make it, anyone else have a better means of making it clear as mud?