Right now if a repo is trending down (from the moving average) it will have a negative score and won't be showed at all.

The algorithm basically looks at the moving average, the new watchers gained and the number of total watchers. It'll calculate the the rate of change in new watchers and compare that against what it expects given the lifecycle of where the repo is at. So a brand new repo adding 150 stars per day will beat out an established repo that adds 250 stars per day.