What is it?

Simply said, try to design your movement so, that you have no peaks in your movement profile. WaveSurfing is designed to move to the lowest danger of your profile, but curve flattening (or just flat movement) is designed to have an equal danger on your whole range. The idea behind it is that even a good PM, GF or DC bot can't easily find a weak spot in your movement. A lot of good bots have such a movement as a secondary movement, only switching to it when they get hit too much on their primary movement. It was very popular in the period of 2004-2007, but the need for it later became much less as both targeting and movement evolved further. I know that f.e. Dookious has such a secondary movement, but I also know that you have to have a very strong gun to trigger it (GresSuffurd can't)