- D1 - Downward force 1
- D2 - Downward force 2
- U - Upward force
D2 and U are a bit more complex, as you can see in the chart below:
U appears to be directly related to the drop ratio, which is the average of the previous 10 drops divided by the number of ideas existing. D2 also shows that relationship, but it is much less clear -- there is another factor I haven't yet discovered.