# FMB alteration based on derivative and noise value

I'm still working on my procedural terrain generation and kind of sniffing ideas from Sean Murray (No Mans Sky programmer). So I watched this GDC17 presentation, but some things just don't make sense to me:

Altering amplitude based on currentGain AND noise value? This just makes all heights (large noise val like 1) do no alteration and the small ones to make the amplitude really tiny. How does that represent Altitude Erosion?

The audio also doesn't help much, has he isn't going into code detail.

Any ideas to makes sense out of this?